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Histological study of the masticatory muscles of the tree shrew showed that 140 muscle spindles are concentrated in the restricted areas of the inner lowermost layer of the horizontal and vertical portions of the temporal muscle (48 and 47 spindles, respectively), the profundus portion of the masseter muscle (42 spindles), and the zygomaticomandibular muscle (3 spindles). The medial and lateral pterygoid muscles are devoid of spindles.

— With the mandible at rest, electromyographic recordings were obtained from the anterior temporal, lateral pterygoid, and digastric muscles with the subjects (A) sitting upright with the head unsupported, (B) inclining backwards with the back-rest at an angle of 45°, and (C) supine. Two minutes after a shift in posture the electrical activity in the anterior temporal muscles was strongest in the upright position, it decreased slightly when the back-rest was at 45°, and was least in the supine position. The digastric muscles showed a pattern similar to that of the temporal. The activity in the lateral pterygoid muscles was strongest when the subject was inclined backwards at 45°. The influence, therefore, of these muscles on the posterior border position of the mandible varies with posture. In all the muscles studied there was least activity when the subject was in the supine position. This would suggest that the supine position is suitable for recording the most retruded position of the mandible. In the intermediate position the lateral pterygoid muscles appear to counteract retrusion.

The clinical nature of the mandibular articulation has been outlined. A fourphase method for diagnosing disorders of the mandibular articulation has been presented. Emphasis has been focused on the facts that the mandibular articulation has the capacity for self-injury, that malocclusion is only one of the many causes of these disorders, and that occlusal balancing techniques are not always indicated as a therapeutic measure. This article emphasizes the importance of discovering all factors which interfere with synchronous and normal movement of the muscles controlling the mandible in addition to muscle spasm as etiologic factors. The importance of abnormal tongue movement and habits has been demonstrated. These and the synergistic movement of the external pterygoid muscles can be etiologic factors in indirect trauma of the mandibular articulation. The coronoid process, carrying the insertion fibers of the temporal muscles, also is an important traumatic factor. Pain from cervical tooth erosion can affect the manner of function of masticatory muscles and give rise to traumas. If the diagnostician approaches such disorders with only the temporomandibular joint in mind, he will be incorrect in the location of the area of the indirect traumas in approximately 60 per cent of patients with mandibular articulation disorders. More often, the lateral head pain reported by the patient arises from injury to the fibers of the muscle at their insertion and from the coronoid process.

P REVIOUS ARTICLES have reported the frequency response requirements encountered in dental electromyography and have outlined a practical means of obtaining electromyograms of jaw muscle activity under optimal conditions. The influence and inadequacy of a single electrode reference on electromyograms was demonstrated and the necessity for using the multielectrode or standard generalized reference was established.l*2 The impetus of electromyography has reached beyond the determination of simple muscle function or dysfunction into the areas of orthodontics, periodontics, prosthodontics, and oral surgery. The electromyographer must be cognizant of the limitations of his instrumentation, and the dental profession likewise must realize the limit of its usefulness. The objectives of this study were (1) to determine the range of variability of muscular activity in jaw movements for a group of patients, (2) to determine the range of variability in a series of electromyograms of one patient, (3) to provide electromyographic data obtained with high-fidelity techniques for future reference, and (4) to provide an analysis of the role played by the external pterygoid muscles in trained (learned) jaw movements. In order to accomplish these objectives, statistical techniques were employed to analyze data derived from the electromyograms.

One complete upper and two complete lower dentures were constructed for each of 15 edentulous subjects. One lower denture accommodated maximal occlusion with the mandible in the muscular position, and the second lower denture accommodated maximal occlusion with the mandible in a protruded position. It was shown that the movement pattern of the mandible can change to accommodate the protruded occlusal position. The dentures constructed to the protruded position caused painful conditions for 10 subjects. The locations of these conditions were (1) the lower denture foundations, (2) the insertions of the masseter muscles, (3) the insertions of the temporal muscles on the coronoid processes, and (4) the external pterygoid muscles. No pain was elicited in the temporomandibular joints. The subjects complained of pain only when it was in the denture foundation. Injury in the other structures was unnoticed. Therefore, the dentist is obliged to examine these muscular structures to determine if the occlusal pattern of dentures is in accord with the movement pattern of the mandible.

SURGICAL repositioning of the mandible for correction of prognathism has become an increasingly popular procedure. Bilateral subeondylar vertical osteotomy is one of many techniques employed. The surgical procedure is relatively easy, and the reported resultP have been excellent. One of the factors which help simplify the technique is the observation that overlapping of the bony edges results in satisfactory osseous union (Fig. 1). Remodeling and healing proceed in a similar fashion, regardless of whether or not the appositional bone is decorticated.5, 6 Interosseous wiring is generally unnecessary if the condylar fragment is positioned lateral to the ramus of the mandible. Because of these observations, the ease and speed with which the operation can be performed have been greatly increased. The bone can be sectioned by either of two techniques : (1) The osteotomy can be performed through a submandibular incision by means of either a nasal saw or dental burs and engine. (2) It can also be performed via a “blind” approach, and the bone can be sectioned with a Gigli saw, It is natural to expect some compensatory changes following surgical intervention. The temporal, geniohyoid, digastric, and mylohyoid muscles are shortened as a result of the mandibular retrusion. The pull of the masseter and internal pterygoid muscles, if not lengthened, is at least altered in direction. The external pterygoid mm&e acts without opposition on the condylar fragment. Changes in the mandibular muscle complex would predispose to secondary mandibular adjustments and remodeling of bone. It was reported earlier* that when the mandible was positioned posteriorly

Electromyography using wire microelectrodes and multichannel recording of the masseter and medial pterygoid muscles demonstrated that in the rabbit, the ipsilateral medial pterygoid muscle actively opposes lateral mandibular movement. In the rat, the ipsilateral medial pterygoid muscle and contralateral masseter muscle oppose the movement. The results correlate with the musculoskeletal arrangements of the two animals.

The shape of the joint surfaces do not govern movements, but these are determined rather by the synergistic action of muscles. The results can be summarized in the following points*: 1. Large variations were found in the patterns of opening and closing, as well as forward and lateral movements because of the laxity of the joint tissues. 2. The instantaneous axis shifted very much during the whole of the opening movement. No single axis could be shown. 3. The angle of opening and the amount of translation of the condyles were calculated. In maximum opening the condyles, as a rule, glided forward some-what further than to the top of the eminentiae. The condyles could be held back actively so that no or only a small amount of translation was effected during the first part of the habitual (natural) opening. 4. The total range of movement of the jaw was seldom utilized during such functions as speech and singing. The opening angles used during speech were generally only from 1.5 to 4.5 degrees. While singing, opening degrees of 4.3 to 8.6 were used as compared to 20 to 30 degrees of total opening. Ten degrees corresponds to 7 to 8 mm. of total condyle translation. Usually, during function, only 2 to 3 mm. of horizontal translation was used. 5. The movement area in the median plane for an anterior point of the mandible did not correspond fully with those described by Henke4 and others. 6. Sagittal translations of the condyles were the largest during lateral movements. 7. The axes for lateral movements were posterior to the condyles. 8. A bodily side shift of the mandible of 2 to 3 mm. was possible. 9. The total angle of rotation (from one extreme lateral position to the other) was calculated to be 30 degrees on the average. 10. Freer lateral movements of the mandible could be performed when it was protruded than could be from its posterior position. 11. Masticatory movements were combinations of opening, forward, and lateral movements. 12. The digastric, geniohyoid, genioglossus, and mylohyoid muscles could cause a lowering of the mandible (opening movement), but not a movement of natural character. 13. The combined effect of the external pterygoid muscles and the afore-mentioned other muscles resulted in an opening movement pattern resembling that obtained in living individuals. The same was true for lateral movements. Many muscles must work together to produce a natural movement. 14. Contraction of the external pterygoid muscles precludes the necessity for extreme shortening of the digastric muscles.

The importance of the growth of the ramus in normal development of the mandible has been recognized for many years. Sarnat [2] has shown the importance of condylar growth experimentally m monkeys. Ricketts [3] has demonstrated the importance of condylar growth in children using a special radiographic technique. No one has proved that prognathic mandibles develop by any mechanism or in a manner greatly different from that of normal mandibles. The prognathic mandible seems to be an expression of quantitative growth rather than of qualitative growth. The size and shape of the alveolar arches of prognathic patients are usually harmonious after their relationship has been corrected. Therefore, the configuration of the prognathic mandible seems to verify the fact that the overgrowth of bone occurs mainly in the ramus, perhaps greatly influenced by condylar growth, as in normal development. For these reasons we believe an operation in the subcondylar region, as close to the origin of overgrowth as possible, properly performed, will produce the best anatomic and physiologic results. Some of the operations used in the treatment of prognathism, such as those involving the overlapping of bone fragments, produce a second deformity while attempting to correct the original deformity. In the application of our technique we have followed sound orthopedic surgical principles in order to correct the prognathism, while producing a minimal disturbance of the normal anatomic position of the associated structures, thereby maintaining or enhancing proper function and improving esthetics. The submandibular approach, or modification thereof, has proved to be a very popular method of entree into the ramus. With proper precautions the two possible complications which have been mentioned in connection with this approach, i.e., injury to the mandibular branch of the facial nerve and parotid fistula, have not proved to be troublesome in a variety of maxillofacial procedures in which this approach has been used. The ever-present possibility of injury to the internal maxillary artery, and the inferior alveolar artery and nerve which exists in operations on the superior portion of the ramus has been eliminated by the presence of the protecting retractor. (Fig. 6.) At the same time accuracy is assured by the use of a precise cutting guide. By careful preoperative study on a stone articulator, as described herein, the relationship of the holes for interosseous wiring can be determined so as to allow for vertical as well as horizontal transposition of the anterior segment which bears the dental arch, and a balanced occlusion with a positively controlled edge-to-edge approximation of the bone fragments can be assured. We believe this new technique of oblique ostectomy of the ramus offers the following advantages: 1. 1. The operative area, as far as we can determine, is as near the anatomic origin of the deformity as possible. 2. 2. The subauriculomandibular approach offers adequate exposure for good visualization of, and access to, the operative area and leaves an inconspicuous scar. 3. 3. There is no oral contamination of the wounds and an aseptic surgical technique is possible. 4. 4. The special self-retaining retractor offers maximum protection to important blood vessels and nerves medial to the site of ostectomy. 5. 5. The preoperative study and precise cutting guide assures accuracy of ostectomy. 6. 6. At the site of operation the ramus is devoid of important blood vessels and nerves, and is relatively free of muscle attachments. 7. 7. The intact body of the mandible and its contiguous tissues are transposed as a unit. 8. 8. The edge-to-edge approximation of bone fragments, stabilized by interosseous wires, reduces the period of immobilization to a minimum and prevents an open bite deformity. 9. 9. The muscles of mastication, with the exception of the external pterygoid muscles, remain attached to the large, denture-bearing, anterior fragment of the mandible which further tends to stabilize the mandible and prevent an open bite deformity. 10. 10. There is minimal disturbance to or enhancement of the functional positions of the muscles of mastication, muscles of the floor of the mouth and the tongue. 11. 11. No teeth are sacrificed or devitalized, and the size of the alveolar arch is not diminished. 12. 12. The acuteness of the gonial angle is increased. 13. 13. The heads of the condyles are retained in their original positions, thereby maintaining the integrity of the intratemporomandibular joint anatomic relationships. 14. 14. By the removal of accurate sections of bone and interosseous wiring, proper alignment of the fragments is maintained. 15. 15. There is no danger of interposing soft tissue between the bone fragments, and healing of bone in the subcondylar region is excellent. 16. 16. The technique requires no elaborate or special method of intermaxillary fixation during the healing process, and is an excellent procedure for edentulous patients.

The muscular activity for various jaw movements has been measured and the total electrical activity of the different muscles compared on a time basis. Based on observed electrical activity and anatomic considerations, it has been concluded that the external pterygoid and suprahyoid muscles were primarily responsible for the uncontrolled opening movement. The masseter and temporal muscles, of those listed, were responsible for the closing movement. The internal pterygoid muscles were not tested. The left external pterygoid muscle was mainly responsible for the movement of the mandible to the right and the right external pterygoid muscle for the opposite movement. Both external pterygoid muscles were responsible for the protrusion of the mandible. The analysis and discussion of the material covering the different sets of dentures will be presented at a later time.

TRAUMATIC maxillary displacement in most cases is posterior and downwards in direction. The maxillary block slides under the curving base of the skull and comes to lie in the open bite position with varying degrees of posterior shift and impacfion. This position is often forcibly maintained by the spastic contraction of the injured pterygoid muscles, the externus pulling the block towards the temporomandibular joint and the internus aided by the masseter muscle pulling the back of the block closer to the descending ramus of the mandible. In the majority of cases this displacement is easily corrected by manipulation under general anaesthesia and the maxilla can be held in the corrected position by a light halo apparatus in the usual way. There are, however, cases that do not respond so readily ; some early ones and nearly all late ones. After a few days an unreduced fracture becomes daily more firmly fixed and continuous traction rather than forcible disimpaction becomes the only way to effect reduction. This has led to the development of a halo which can exert pressure on the displaced maxilla in any direction through the medium of two self tapping screws inserted in the premaxilla (Grocott and Wilson, I972). A number of different frame types were tried ; that finally selected is an adjustable but rigid structure which gives access to the vault of the skull and is comfortable to the patient in bed. Necessarily this means a heavier structure than the standard halo but the weight has been progressively reduced to about 350 gm. by the use of light alloy. The frame (Fig. I) consists of a horizontal metal platform conforming in contour to the forehead and adjustable side arms pivoted at the outer ends of the platform. The platform itself carries 2 insulating blocks or bushes which in turn carry the stainless steel frontal screw pins (Fig. 2). In the centre of the platform is a ball or universal joint which forms the pivot for a vertical extensible beam dropping down to the maxillary attachment. The joint is fully adjustable by screws so that the beam may rotate and swing in an antero-posterior or lateral direction in any combination. The pivoted side arms are adjustable by screws and can be locked in final adjustment by nut and bolt. They are drilled and can carry vertical arms if desired. The posterior ends are slotted to carry the nylon insulated bushes which hold the mastoid screw pins. It is very advantageous to have as short a length of screw pin as possible between the frame and the skull. This ensures greater rigidity and obviates movement of the pin carriers or bending of the pins under stresses tangential to thc ~t'ull The skull strong points selected for insertion of the pins are on each temporal ridge area and each mastoid area at the level of the floor of the anterior fossa. The pin holders are first adjusted for position and then the angles are set for radial contact with the bone. The clamp is applied so that the mastoid pins register just in front of the mastoid suture line with the occipital bone; there is a depression here which is easily felt behind the ear. The lateral arms are tightened until the pins register against the bone in the depression. The pins, which are hollow ground, are screwed in a few turns until the whole clamp can be hinged up and down with the frontal pins just clearing the forehead. The frontal pins are now screwed home to register just above the superciliary ridges and just medial to the temporal ridges of the frontal bone. When these and the mastoid pins are tightened the platform is solidly fixed to the base of the skull.

V ARIOUS OPINIONS ARE HELD by different investigators as to the functional importance of the anatomic structures which take part in limiting retrusion of the mandible. Arstadl points out that in other joints, of the digits for example, limitation of motion is due to the joint capsule and re-enforcing ligaments. In the temporomandibular joint, the capsule is slack and flaccid, so much so that it is not injured when the joint is dislocated. The sphenomandibular and stylomandibular ligaments, by their position, cannot prevent the condyles from sliding dorsally. In 1933,2 Arstad maintained that the anterior part of the joint capsule supports the retruded position. In 1954,l however, he described some collateral capsular ligaments as being responsible for limited retrusion of the condyles. Steinhardt3 reported in 1934 that the external pterygoid muscle limits retrusion of the condyles. In 1957, however, Steinhardt4 agreed with Arstadl that the collateral capsular ligaments of the temporomandibular joint are responsible. He also said that the muscles are important but did not elaborate. Siche+a stated that one function of the external pterygoid muscle is to hold the condyle and disc against the posterior slope of the articular eminence while the teeth are in contact, Markowitz and Gerry? and other investigators state that the function of the external pterygoid muscles acting together is to cause protrusion of the mandible. The role muscles play in limiting retrusion of the condyles was contradicted by the work of Aprile and Sai.zar3 when they obtained symphyseal angle tracings from specimens with the joints intact but with the muscles removed. The work of PosseltQ also nullified the theory that muscles are the effecters which limit posterior retrusion of the condyles. Posselt obtained Gothic arch (stylus) tracings from anesthetized living persons under the influence of curare. Ferrein,lO Breuer,ll and ReeC reported that the retruded position of the condyles is held by the temporomandibular ligament. Kirk13 was of the opinion that the denture bears the main stress of mastication and prevents the backward thrust of the condyles. Robinson14 reported that the mandibular condyle in its most retruded position is not normally situated at the bottom of the mandibular fossa but is closer to the posterior part of the tuberculum articulare.

The evidence which I have compiled indicates that the bulk of temporomandibular cases, of indirect traumatic origin, can be divided into two pathologic categories: (1) those occurring within the joint structure including contusion, osteoarthritis, and occasionally rheumatoid arthritis in this order of frequency, and (2) those occurring periarticularly with functional alterations of the external pterygoid muscles. The two categories may occur solely or concomitantly. The temporomandibular joint is extremely flexible and normally is the victim rather than the active guide of the opening tilt. This joint and the external pterygoid muscle, when injured or strained, create an area of hyperesthesia or oversensitiveness and are capable of producing considerable psychogenic disturbances. The degree and chronicity of the involvement depend upon the nature of the injury to the external pterygoid muscle, probably the superior fasciculus of this muscle. The initial injury can arise from compression by the condyle head, either in an anterior glide, lateral rotation, or during the opening tilt, or to an injury, strain, or fatigue within the external pterygoid muscle. The nature of the problem is different with a posteriorly displaced condyle, which produces more tearing, than with a condyle in an anterior position, which is a compression problem. Any diagnosis of temporomandibular complaints must start at the level of the joint itself, and not at the level of the dental arches. There are other causes of pain in this area, such as constitutional, the external acoustic meatus, fluid collecting in the infratemporal fossa, the coronoid process, and the posterior and insertion fibers of the temporal muscles. Any one of these could he the sole cause for pain in this area. Fig. 6 shows in outline a minimum spectrum of thought in diagnosing these cases. This is a simple, specific technique for diagnosing these joint complaints, for ascertaining the degree of involvement, and for determining independently of the patient's report whether or not therapeutic measures have been successful. While occlusal therapy is still our most effective therapeutic measure, I believe it is apparent that we must use other aids in some of these cases. For example, I have used 2 per cent Novocain as a therapeutic aid and to help differentiate the psychogenic cases. In some, I got a cure unexpectedly, i.e., relief from pain. More recently, I have used hyaluronidase, and Hydrocortone in well-selected cases. I feel that injection therapy can be an important adjunct, but it should not be used empirically. I have presented only the core of a differential diagnostic technique which has been guided by my early dissections. Many phases of this outline can be developed more extensively. I have found this approach to be very satisfactory, and without the confusion attending most discussions I have heard or read. Whenever a seemingly new symptom develops in a patient's temporomandibular joint problem, I have found the solution by thinking as directed in this article, and I have discovered the explanation in normal pathologic or physiologic terminology. I believe this to be an effective, concrete technique for diagnosing temporomandibular and periarticular disturbances.

An analysis of temporomandibular articulation reveals that: 1.1. The interarticular fibrocartilages prevent direct contact of the condyles with the surfaces of the fossae during all mandibular movements. This precludes any possible condylar guidance by the shape of the bony fossae. 2.2. Physiologic rest position is a definite mandibular position which maintains airways and is controlled by the neuromuscular system. 3.3. Centric relation is a definite mandibular position which is assumed in the function of deglutition. This position is the starting point of all mandibular movement, because the mandible continually returns to this position in the course of the swallowing reflex. 4.4. Lateral mandibular movement is a rectilinear movement forward toward the midline and parallel to the occlusal plane. This movement is not translation. 5.5. Protrusion of the mandible is a rectilinear forward movement parallel to the occlusal plane. Protrusion is translation. 6.6. The horizontal axes of certain mandibular movements are in the necks of the condyles. 7.7. The Bennett movement is the lateral shift of the mandible coincident with lateral mandibular movement.

Satisfactory application of the electromyographic method as a research tool has not been made as yet in the field of dentistry. Very few reports, therefore, are available for understanding the functions of orofacial muscles from this point of view.The aim of this report is to describe the findings on the functions of masticatory muscles which were obtained through studying their myograms and, if necessary, the mandibular movements as well as the bite pressure recorded simultaneously.The results and conclusoins are summarized as follows:1) The discharge of the neuromuscular units composing the group of masticatory muscles was of a pattern similar to those of the limb muscles. In some units, the magnitude of spike discharge was measured a few milivolts. The spike duration was 2-5 milisec., a little shorter than that in the limb muscles and there were found some units with their discharge frequencies of more than 70 C.P.S.. From these results it is possible to consider that the innervation ratio for the masticatory muscles is smaller than that for the limb muscles.2) In the rest position of the mandible, when the Frankfort plane of the subject was kept to be in paralell with the horizontal, no discharge was recorded from all the masticatory muscles under examination.3) Determination was made of the rest position of the mandible through studying the myograms of the masseter, the temporal and the digastric muscles. The rest position thus obtained was 2.6 mm as to the masseter and the temporal, and 6.1 mm to the digastric muscle, having the clearance of 3.5 mm as their mean value. From this finding, it is probable that the rest position of the mandible can directly be determined by way of electromyography.4) The followings are the findings on functions of muscles as to the mandibular movements which were obtained from their myograms in the movements in six directions (i.e., upward, downward; forward, backward; leftward, rightward) where the rest position was taken as starting position for the movements.a) The elevation of the mandible is accomplished by the contraction of the masseter, the temporal and the internal as well as external pterygoid muscles of both sides.b) The digastric muscles of both sides take parts in the forward and backward movements. The masseter and the internal pterygoid muscles participate in the forward movement, while the temporal and the external pterygoid in the backward.c) To the lateral muandibular movement toward one side contribute the masseter and the internal pterygoid muscles of the same side but also the temporal and external pterygoid muscles of the other side and further the digastric muscles of both sides. Further investigation by way of electromyography is necessary to make more clearer the functions of the internal and the external pterygoid muscles as to the mandiblular movements.5) The followings are the informations obtained from simultaneous recording of the mandibular movements with reference to the discharge of the muscles involved.a) In biting, the superficial and the deep head of the masseter muscles function together, but in the case of the mouth closing the superficial alone.b) The anterior, the middle and the posterior belly of the temporal muscle function together in biting, but in closing the mouth there functions the middle belly in chief.c) The digastric and the sternohyoid muscles take part in the mouth opening. But the discharge in the sternohyoid muscles was not observed before the opening advanced to a great extent, so that the suprahyoid muscles seem to participate in the action of the mouth opening, partly aided by the infrahyoid muscles.d) At the maximal opening of the mouth the masseter and the temporal muscles showed discharges which are considered to be elicited by their proprioceptive reflex. And this phenomenon is likely to indicate that there exist some muscle spindles in the masticatory muscles.

Structure and functioning of the kinetic apparatus of the mallard (Anas platyrhynchos L.), and degrees of freedom in movements and muscle activities of a functioning kinetic apparatus were investigated. Origins and insertions of the relevant muscles (DAVIDS, 1952) and ligaments, the courses of the muscle fibres, the aponeurotic fibres, the ligaments and ligament fibres and the structure of the bony elements and joints as well as their range of movement, are described. Electromyography and cinematography were used to investigate the muscle activities and movements in three functions of the kinetic apparatus: the straining of fine-grained food in water, the pecking of dry coarse-grained food, and drinking. In relatively constant periods of repetitive actions average pictures of the movements and muscle activities were made for straining. For drinking, the shortest scene was taken as a basis for constructing an average picture. The activity of a muscle was quantified relatively by dividing the difference between the minimal and maximal amplitudes in three steps. This method results in average graduated block diagrams, which were prepared for 18 muscles during straining and drinking and combined with the average synchronized movements. These movements concerned the maxilla and the mandible, with reference to the skull and each other, and the distance between the tips of the beak. Straining is described on the basis of an α and a β cycle model and drinking by two other models: the odd and even postphase cycle model. The movements of the quadrate proved to be critical in these models, since these movements determine the position of the fulcrum between the quadrate and the mandible. Five groups of muscles can be distinguished, delivering respectively: 1. power for opening, 2. guiding for opening, 3. 'streamlining' for the transition from opening to closing, 4. power for closing, 5. guiding for closing. The assumption of coupled kinesis proposed by VON KRIPP (1933a) and BOCK ( 1964) is partially applicable here to the first part of opening the beak. The closing system can be loaded by the tension of active skull adductors and unloaded by the effect of the quadrate adductors and of the ventrolateral and ventromedial pterygoid muscles on the position of the fulcrum between mandible and quadrate. This can result in very high acceleration of movement. The same is shown for the opening system, i.e., loading by the depressors and unloading by the effect of depressors and protractors on the position of the fulcrum. The system is double-coupled, which makes very rapid, guidable, and repeated movements possible. The models for drinking indicate that the guiding system can lead to two modes of application of the forces of the power muscles. An attempt is made to specify quantitatively the α model in a first approximation. The results of this quantification improve the qualitative α model. A distinction is made between "guiding" and "power" muscles on the basis of the differences in structure and functioning of the skull adductors and quadrate adductors in the α and β models. The differences between the characteristics of these muscles are indicated and partially explained. To this end, for in vivo active pinnate muscles the length-tension diagram for in vitro tetanically activated muscles of HILL (1953) is modified such that the maximum available total contraction force can be realized in slightly lengthened pinnate muscles. In addition, the conclusion reached by ERNST (1963) concerning the absence of storage of elasticity in tetanically activated pinnate muscles, was modified for in vivo active pinnate muscles such that for a very short time during very fast stretching and motoneuronal activity, elastic energy as such is stored and is very briefly available. When the patterns for straining, pecking, drinking, and remaining plural and singular actions are compared, in a first approximation the realization of these actions requires only a few more basic macrovariations than the 4 models mentioned. It is necessary to quantify these basic macrovariations to obtain an insight into the totality of mechanical kinetic demands made by the functioning of the feeding apparatus.

There seems to be a scarcity of references in the literature on abscesses of the pterygomaxillary or infratemporal fossa. In this paper I therefore wish to review the literature and to report a case. The pterygomaxillary, infratemporal or zygomatic fossa is the space bounded laterally by the ramus of the mandible, medially by the lateral wall of the pharynx and pterygoid process of the sphenoid bone, anteriorly by the zygomatic surface of the superior maxilla and superiorly by the lower surface of the greater wing of the sphenoid and the adjacent temporal bone. The posterior limit of the space is represented by a plane passing directly medialward from the posterior border of the ramus of the mandible to the pharynx. The contents are the internal and external pterygoid muscles, the internal maxillary artery with its branches and companion veins, the pterygoid plexus of veins, the inferior maxillary nerve and its

The suggestion that the medial angulation of the external pterygoid muscles will effect a contracting action upon the two halves of the jaw was made by Grunewald in 1921.1 More recently, De Brul and Sicher2 demonstrated lines of stress near the symphysis of the mandible by squeezing the condyles together with finger pressure at the insertion of the external pterygoids.* They reported a distortion (in the living) between the last mandibular molars which was up to a mm. less when the jaw was forcefully protruded than when it was at rest. In an effort to characterize quantitatively the decrease in mandibular width from the rest position to a forced protrusive and a wideopen position the following study was undertaken.

Wear facets on molars of the Eocene primate Adapis magnus are described. Striations on these wear facets indicate three separate directions of mandibular movement during mastication. One direction corresponds to a first stage of mastication involving orthal retraction of the mandible. The remaining two directions correspond to buccal and lingual phases of a second stage of mastication involving a transverse movement of the mandible. The mechanics of jaw adduction are analysed for both the orthal retraction and transverse stages of mastication. During the orthal retraction stage the greatest component of bite force is provided by the temporalis muscles acting directly against the food with the mandible functioning as a link rather than as a lever. A geometrical argument suggests that during the transverse stage of mastication bite force is provided by the temporalis muscles of both sides, the ipsilateral medial and lateral pterygoid muscles, and the contralateral masseter muscle.

Electromyograms of the principal muscles of mastication, the masseter muscle, the three parts of the temporalis muscle, the medial and lateral pterygoid muscles and the anterior belly of the digastric muscle, have been taken on medical students in the open-close-clench cycle. A microphone attached to the skin over the zygoma was used to determine initial tooth contact. Recordings from the masseter and the anterior part of the temporalis muscles were taken using surface electrodes. The digastric muscle was examined with both surface and needle electrodes; needle electrodes were used for the remaining muscles. A period of inactivity of the mandibular elevators of approximately 13 msec duration was found in each cycle of every individual. This inactive phase commenced some 13 msec after initial tooth contact (“latent period”). Variable activity of the lateral pterygoid muscle and the anterior belly of the digastric muscle was seen in the closing phase of the open-close-clench cycle. The lateral pterygoid muscle showed an inactive period similar to that of the mandibular elevators following tooth contact, whereas the anterior belly of the digastric muscle showed a localized increase of activity at that time. Similar results followed tapping of a tooth when the jaws were slightly apart. It is believed that this cessation of activity of the mandibular elevators represents an active reflex inhibition rather than a passive mechanical cessation of activity and is associated with reciprocal excitation of the digastric muscle. End organs appear to be located primarily in the periodontal membrane. The probable neurological pathway is discussed.

Neuromuscular spindles were identified in each of the six lateral pterygoid muscles of the guinea pig examined in this study. Their structure complied with the usual description but their number was sparse considering the size of the muscle. The lengths and breadths of the spindles were measured and the number of their intrafusal muscle fibres recorded.

Pressure of at least 300 g, when applied rapidly to the labial or lingual surface of a maxillary central incisor of lightly anesthetized decerebrate rabbits, caused the mandible to swing to the opposite side, where it would remain for 2–10 sec if the pressure were maintained. On the contralateral side, the zygomaticomandibular and anterior temporal muscles showed the highest levels of maintained activity. The digastric muscle was sometimes active at the start of stimulation while the superficial masseter became active later and returned the mandible to the midline. The external pterygoid muscles of both sides fired at the start of stimulation. Other muscles ipsilateral to the applied stimulus were inhibited. Section of the infraorbital nerve above, but not below, its union with the anterior superior alveolar nerves abolished the reflex response when pressing on the incisor of that side. A similar reflex could be elicited by electrical stimulation of the pericoronal incisal gingivae or adjacent palatal mucosa if the stimulus frequency was greater than 7 Hz. Using a stimulus of 30 Hz, 6–15 v, 1 or 2 msec, reflex activity began in the zygomaticomandibular and anterior temporal EMG 80–210 msec from the beginning of the pulse train. Evoked activity followed each stimulus with a latency of 9.0–16.0 msec. This reflex could act during chewing to modify the basic central masticatory pattern.

In cases in which bilateral horizontal ramisection is indicated for prognathism, it is believed that open bite can be prevented by reattachment of the masseter and internal pterygoid muscles bilaterally. This prevents stretching of these muscles. Four such cases have been successfully treated with a follow-up as long as ten years in one case with no open bite in any case. One case is discussed in which this procedure along with a pharyngeal flap and tracheotomy for anesthesia were simultaneously performed.

Five rhesus monkeys were sacrificed after their jaws were propped wide open. Histologic sections showed the following: 1. The interarticular discs had moved completely in unison with the condyle in mandibular opening. 2. There was no indication of pressure to the peripheral parts of the discs. 3. The external pterygoid muscles did not seem to play an active role in the anterior positioning of the discs. 4. There was no histologic evidence of an anterior joint capsule. 5. Large vascular and open synovial spaces appeared in the retrocondylar soft tissues. 6. Superficial soft tissues including part of the parotid glands had been pulled into the retrocondylar space during the opening of the mandible.

Article abstract A 68-year-old woman had asymmetrical but bilateral enlargement of the temporalis, masseter, and pterygoid muscles that had begun at the age of 30 and was for the most part progressive. The disorder has been confined to the muscles of mastication; during 15 years of observation no muscles elsewhere have been affected. Histopathologic features are those of a mild myopathic illness, termed branchial myopathy because of the embryologic origin of the muscles concerned. The etiology of this benign disorder is not known, if indeed there is a solitary cause underlying all cases. Surgical treatment may be prompted by mechanical or cosmetic factors.

A description of the pterygoid region of the cat is presented. The findings indicate that the attachments of the pterygoid muscles differ from those described in the English language. The corresponding regions of the sheep, goat, and monkey have also been described, and some comparative anatomical features noted.

The illustration gives the exact size of two of several pieces of wood that were removed from the infratemporal space, where they had lodged for four months following an injury. The infratemporal fossae is the open space behind the maxillae, under cover of the tendon of the temporalis, the coronoid process and the ramus of the mandible and the masseter muscle. It contains the pterygoid muscles and the internal maxillary artery among the most important. REPORT OF CASE A man, aged 51, admitted to Los Angeles General Hospital, Feb. 7, 1923, was employed as a foreman on a steam pile-driver when a large splinter, several feet long, broke off, striking on the side of the face. Examination disclosed a deep laceration over the right side of the face. The tissues over the malar eminence were macerated and detached. The lids of the right eye were torn and contused, the lower

The resting lengths of fibres of adult rat temporalis, masseter and pterygoid muscles were measured directly after maceration and teasing. They were fairly uniformly about 8 mm long, except the medial pterygoid, which was about 5 mm long. Maximum excursions of the muscles were estimated by comparing the distance between their origins and insertions on radiographs of the jaw in extreme positions of movement and more reliably by measuring the difference between the fully contracted and extended fibre and sarcomere lengths. They varied from about 30–55 per cent of the extended fibre length. An index of cross-sectional area (weight/fibre length) showed the ratio of strength of masseter, temporalis, medial and lateral pterygoid to be about 10:5:2.5:1. Male muscles were slightly larger and stronger than female.