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The epidural space of the American alligator (Alligator mississippiensis) is largely filled by a continuous venous sinus. This venous sinus extends throughout the trunk and tail of the alligator, and is continuous with the dural sinuses surrounding the brain. Segmental spinal veins (sl) link the spinal venous sinus (vs) to the somatic and visceral venous drainage. Some of these sl, like the caudal head vein along the occipital plate of the skull, are enlarged, suggesting more functional linkage. No evidence of venous valves or external venous sphincters was found associated with the vs; the relative scarcity of smooth muscle in the venous wall of the sinus suggests limited physiological regulation. The proatlas (pr), which develops between the occipital plate and C1 in crocodylians, is shaped like a neural arch and is fused to the dorsal surface of the vs. The present study suggests that the pr may function to propel venous blood around the brain and spinal cord. The vs effectively encloses the spinal dura, creating a tube-within-a-tube system with the (smaller volume) spinal cerebrospinal fluid (CSF). Changes in venous blood pressure, as are likely during locomotion, would impact dural compliance and CSF pressure waves propagating along the spinal cord., (© 2024 The Authors. The Anatomical Record published by Wiley Periodicals LLC on behalf of American Association for Anatomy.)

The proatlas, a bone located between the skull and the neural spines of the cervical vertebrae, is best known from reptiles. Most previous studies of the proatlas have centered on its developmental, debating the relationship between the proatlas and the cervical neural arches. The present study was intended as a description of the proatlas in the American alligator (Alligator mississippiensis) and an experimental test of its hypothesized role in venous blood and cerebrospinal fluid (CSF) distribution. In Alligator, the proatlas is chevron-shaped; ventrally it has a loose connection to the dorsal surface of the first cervical vertebrae, dorsally it has a robust elastic tissue tether on the otoccipital and supraoccipital bones. The ventral surface of the proatlas parallels the dorsal margin of the foramen magnum and rests on the dorsal surface of the spinal venous sinus. Experimental manipulation of the proatlas demonstrated that displacement of the proatlas causes pressure changes in both the spinal venous sinus and the enclosed spinal CSF. The results of this study represent the first demonstration of an explicit functional role for the proatlas, the circulation of fluids between the cranial and spinal compartments of the central nervous system., (© 2024 The Authors. Journal of Morphology published by Wiley Periodicals LLC.)

In humans and most mammals, there is a notch-like portal, the foramen of Luschka (or lateral foramen), which connects the lumen of the fourth ventricle with the subdural space. Gross dissection, light and scanning electron microscopy, and μCT analysis revealed the presence of a foramen of Luschka in the American alligator (Alligator mississippiensis). In this species, the foramen of Luschka is a notch in the dorsolateral wall of the pons immediately caudal to the peduncular base of the cerebellum, near the rostral end of the telovelar membrane over the fourth ventricle. At the foramen of Luschka there was a transition from a superficial pia mater lining to a deep ependymal lining. There was continuity between the lumen of the fourth ventricle and the subdural space, via the foramen of Luschka. This anatomical continuity was further demonstrated by injecting Evans blue into the lateral ventricle which led to extravasation through the foramen of Luschka and pooling of the dye on the lateral surface of the brain. Simultaneous subdural and intraventricular recordings of cerebrospinal fluid (CSF) pressures revealed a stable agreement between the two pressures at rest. Perturbation of the system allowed for static and dynamic differences to develop, which could indicate varying flow patterns of CSF through the foramen of Luschka., (© 2023 The Authors. Journal of Anatomy published by John Wiley & Sons Ltd on behalf of Anatomical Society.)

The different velocities of sound (pressure waves) in air and water make auditory source localization a challenge for amphibious animals. The American alligator (Alligator mississippiensis) has an extracolumellar cartilage that abuts the deep surface of the tympanic membrane, and then expands in size beyond the caudal margin of the tympanum. This extracolumellar expansion is the insertion site for two antagonistic skeletal muscles, the tensor tympani, and the depressor tympani. These muscles function to modulate the tension in the tympanic membrane, presumably as part of the well-developed submergence reflex of Alligator. All crocodilians, including Alligator, have internally coupled ears in which paratympanic sinuses connect the contralateral middle ear cavities. The temporal performance of internally coupled ears is determined, in part, by the tension of the tympanic membrane. Switching between a "tensed" and "relaxed" tympanic membrane may allow Alligator to compensate for the increased velocity of sound underwater and, in this way, use a single auditory map for sound localization in two very different physical environments., (© 2023 The Authors. The Anatomical Record published by Wiley Periodicals LLC on behalf of American Association for Anatomy.)

Background: Dural compliance influences the shape and magnitude of the cerebrospinal fluid (CSF) pulsations. In humans, cranial compliance is approximately 2× greater than spinal compliance; the differential has been attributed to the associated vasculature. In alligators, the spinal cord is surrounded by a large venous sinus, which suggests that the spinal compartment may have higher compliance than is found in mammals., Methods: Pressure catheters were surgically implanted into the cranial and spinal subdural spaces of eight subadult American alligators ( Alligator mississippiensis ). The CSF was propelled through the subdural space by orthostatic gradients and rapid changes in linear acceleration., Results: CSF pressure recordings taken from the cranial compartment were consistently, and significantly, larger than those taken from the spinal compartment. After the myodural bridge of Alligator was surgically released, the asymmetry in CSF pressure was decreased., Conclusion: Unlike the situation in humans, the spinal compartment of Alligator has greater compliance than the cranial compartment, presumably due to the presence of the large spinal venous sinus surrounding the dura. The change in CSF pressures after myodural surgical release supports the hypothesis that the myodural bridge functions, at least in part, to modulate dural compliance and the exchange of CSF between the cranial and spinal compartments., Competing Interests: There are no conflicts of interest., (Copyright: © 2023 Surgical Neurology International.)

Secondary neurulation is a common feature of vertebrate development, which in non-mammalian and non-anuran vertebrates, results in the formation of a caudal spinal cord. The present study was undertaken to describe the terminal end of the caudal spinal cord in a crocodylian, a group chosen for their unique status of a living-tailed archosaur. The caudal spinal cord of Alligator mississippiensis terminates near the intervertebral joint between the fourth and fifth terminal vertebrae. Prior to this termination, the dorsal root ganglia get proportionately larger, then stop before the termination of the spinal cord; and the gray matter of the spinal cord is lost producing an unusual morphology in which an ependymal-lined central canal is surrounded by only white matter which is not divided into a cauda equina. The inner layer of the meninges (the pia-arachnoid) courses over the distal end of the spinal cord and forms a ventral attachment, reminiscent of a very short Filum terminale; there is no caudal cistern. The dura extends beyond the termination of the spinal cord, continuing for at least the length of the fourth terminal vertebra, forming a structure herein termed the distal meningeal sheath. During its course, the distal meningeal sheath surrounds a mass of mesothelial cells, then terminates as an attachment on the dorsal surface of the vertebra., (© 2022 The Authors. The Anatomical Record published by Wiley Periodicals LLC on behalf of American Association for Anatomy.)

Biomechanically, the patellofemoral joint is one of the most complex human articulations and a common source of pain for active adults and adolescents, particularly females.1-4 Patellofemoral disorders account for 20%-40% of all knee problems seen in family practice, sports medicine, and orthopedic clinics.1, 3-5., (Copyright 2023 by the Missouri State Medical Association.)

One of the distinctive features of the Crocodylia is the presence of a superficial meatal chamber the aperture of which is regulated by two earflaps. The movements of the upper earflap have been detailed by multiple workers, however, the mechanics of the lower earflap remain unresolved. The present study was undertaken to document the mechanics of the lower earflap in the American alligator, Alligator mississippiensis, and to explore the functional bases of coordinated movements between the lower earflap and lower eyelid in this species. This anatomical system was examined using a combination of fresh dissection, histology, and micro-CT analyses applied to post-embryonic specimens. The rostral margin of the lower earflap is tightly bound to a block of dense connective tissue herein termed the orbitalauricular chord. The orbitalauricular chord is anatomically distinct from both a ligament and a tendon. The dorsal surface of the orbitalauricular chord is attached to a slip of the levator palpebra, while the ventral surface is attached to a slip of the depressor palpebra. These attachments produce a simple mechanism for the elevation and depression of the lower earflap, and thus the opening and closing of the meatal aperture. The caudal surface of the orbitalauricular chord has connective tissue links to the rostral margin of the lower earflap. The morphology of the orbitalauricular chord appears to explain both the mechanics of the lower earflap and the functional coupling between the lower eyelid and lower earflap., (© 2022 The Authors. Journal of Anatomy published by John Wiley & Sons Ltd on behalf of Anatomical Society.)

This study was undertaken to document how the cerebrospinal fluid (CSF) pressure varied during movements and physiological activities. Using surgically implanted pressure catheters; the CSF pressure was recorded from sub-adult American alligators ( Alligator mississippiensis ) under anesthesia and post-recovery. Pressures were recorded during physiological activities (the cardiac cycle; passive and active ventilation); manual manipulation of the anesthetized animals (foot sweeps; tail oscillations; and body bends); as well as voluntary movements post-recovery (changes in body tone; defensive strikes; and locomotion). The CSF pulsations associated with the cardiac cycle had the lowest mean amplitude (3.7 mm Hg); during active ventilation and defensive strikes; the alligators routinely generated CSF pressure spikes in excess of 100 mm Hg. The recorded CSF pressures appear to be caused by a variety of mechanisms including vascular pressure; fluid inertia; and possible physical displacement of the spinal cord. The results of the study suggest that any model of CSF dynamics or perfusion should incorporate the episodic high-pressure CSF pulsations associated with movement.

The American alligator (Alligator mississippiensis) has a postpulmonary septum (PPS) that partitions the intracoelomic cavity. The PPS adheres to the capsule of the liver caudally and to the visceral pleura of the lung cranially; the ventrolateral portions of the PPS are invested with smooth muscle, the remainder is tendinous. Differential pressure transducers were used to record the intrathoracic (ITP) and intraperitoneal (IPP) pressures, and determine the transdiaphragmatic pressure (TDP). Each ventilatory pulse resulted in a pulse in ITP and a significantly lower pulse in IPP; meaning that a TDP was established, and that the pleural and peritoneal cavities were functionally isolated. The anesthetized alligators were tilted 30° head-up or head-down in order to displace the liver. Head-up rotations caused a significant increase in IPP, and a significant decrease in ITP (which became negative); head-down rotations produced the opposite effect. During these rotations, the PPS maintained opposite pressures (positive or negative) in the pleural and peritoneal cavities, and established TDPs greater than have been reported for some mammals. Two types of "breaths" were recorded during these experiments. The first was interpreted as a contraction of the diaphragmaticus muscle, which displaces the liver caudally; these breaths had the same effect as the head-up rotations. The second type of breath was interpreted as constriction of the thoracic and abdominal body walls; this type of breath produced pronounced, long-duration, roughly parallel, increases in ITP and IPP. The smooth muscle within the PPS is suggestive of higher-order adjustment or tuning of the PPS's tensile state., (© 2021 American Association for Anatomy.)

The myodural bridge, that is, skeletal muscle fibers attaching to the cervical dura mater, has been described from a variety of mammals and other amniotes. To test an earlier assumption about the presence of the myodural bridge in snakes, a comparative study was designed using a group of Colubrine snakes. Serial histological sections revealed no evidence of the myodural bridge in any of the snakes examined. Further analyses, including histology, computed tomography (CT), and micro-CT imaging of other distantly related snakes, also turned up no evidence of a myodural bridge. The close apposition of adjacent neural arches in snakes may preclude muscle tendons from passing through the intervertebral joint to reach the spinal dura. It is hypothesized that the myodural bridge functions in the clearance of the cerebrospinal fluid (CSF) by creating episodic CSF pressure pulsations, and that snakes are capable of creating equivalent CSF pressure pulsations through vertebral displacement., (© 2021 Wiley Periodicals LLC.)

In the viper boa ( Candoia aspera ), the cerebrospinal fluid (CSF) shows two stable overlapping patterns of pulsations: low-frequency (0.08 Hz) pulses with a mean amplitude of 4.1 mmHg that correspond to the ventilatory cycle, and higher-frequency (0.66 Hz) pulses with a mean amplitude of 1.2 mmHg that correspond to the cardiac cycle. Manual oscillations of anesthetized C. aspera induced propagating sinusoidal body waves. These waves resulted in a different pattern of CSF pulsations with frequencies corresponding to the displacement frequency of the body and with amplitudes greater than those of the cardiac or ventilatory cycles. After recovery from anesthesia, the snakes moved independently using lateral undulation and concertina locomotion. The episodes of lateral undulation produced similar influences on the CSF pressure as were observed during the manual oscillations, though the induced CSF pulsations were of lower amplitude during lateral undulation. No impact on the CSF was found while C. aspera was performing concertina locomotion. The relationship between the propagation of the body and the CSF pulsations suggests that the body movements produce an impulse on the spinal CSF.

Introduction: The anterior cruciate ligament (ACL) prevents the anterior translocation and medial rotation of the tibia against the femur. It is typically composed of dense regular connective tissue (DRCT), small amount of loose connective tissue, little vasculature, and few nerve endings. The objective of the current study was to evaluate the details of histological changes in ACLs of patients with clinically diagnosed osteoarthritis (OA)., Materials and Methods: The ACLs of six patients undergoing total knee replacement because of OA (OA group) were compared with 16 normal ACLs from cadavers (control). The ACLs were analyzed for tissue composition and number of blood vessels across the full length and thickness of the ligament. Percentages for areas of DRCT, fibrocartilage, degenerative tissue, and vasculature were calculated. Tissue composition and relative number of blood vessels were compared between groups., Results: The proportion of DRCT to non-DRCT was significantly smaller in the OA group than the control group (p < .001); non-DRCT included degenerative connective tissue and fibrocartilage. The number of blood vessels to area was greater in the OA group than the control group (p = .002). Six of control (37.5%) and five of OA ACLs (83%) showed areas of calcification., Conclusions: These results indicate that inflammatory processes contributing to OA in the knee cause changes in the composition of the ACL that lead to destruction of collagen bundles, increased vascularization, calcification, and formation of fibrocartilage-like tissue inside the ligament. These changes make ligament-retaining total knee arthroplasty a less beneficial option for knee repair., (© 2020 Wiley Periodicals, Inc.)

The crocodilian naris is regulated by smooth muscle. The morphology of this system was investigated using a combination of gross, light microscopic, and micro-CT analyses, while the mechanics of narial regulation were examined using a combination of Hall Effect sensors, narial manometry, and electromyography. Alligator mississippiensis, like other crocodilians, routinely switches among multiple ventilatory mechanics and does not occlude the nares during any portion of the ventilatory cycle. In a complex that is unique among vertebrates, a single block of smooth muscle functions in dilation when active, and in constriction when passive. The alligator nares may include one of the best examples of a muscle that functions in "pushing" as well as "pulling." The central muscle for narial regulation, the dilator naris, can legitimately be viewed as its own antagonist., (© 2020 Wiley Periodicals, Inc.)