An essay on the anatomy of the gibbon (Hylobates agilis), with notes on comparative anatomy

The kindness of Professor Cunningham has enabled me to carry out the dissection of a gibbon in his possession during the past Winter Session. The dissection was carried out in the Anatomical Department of Edinburgh University. I have, at the risk of seeming tedious, described the details of the muscles fully under the headings origin, insertion, nerve supply, structure, and relations. My reason for doing this is the lack of evidence, positive or negative, on certain points of interest in this dissection. which I looked for in the works of others. Under the heading of comparative anatomy the observations of other writers on the anatomy of the anthropoid apes have been fully referred to: as far as possible only those points in the lower animals have been noted which seemed to throw light on the anatomy of the apes and man. Many of the points which have arisen during the investigation I have sought to elucidate by observations made in the Museum of the University and in the Natural History Department of the Museum of Science and Art in Chambers Street, and by dissections made on some of the lower animals, such as the cat, mole, and fish. Although fully sensible of the futility of propounding theories on comparative anatomy from the results of short investigation, I have ventured to express opinions on certain views ,which this dissection seemed to deal with directly. As far as possible the points have been illustrated, the drawings are with the exception of three, the results of my own work. In the muscular system the peculiar continuation of the muscular sheets of the trunk on to, and down the fore limb have been noted in connection with the habits of the animal. The musculature of the forearm and hand has been fully considered, I believe the right interpretation to have been put upon certain anomalous muscles found in the palm of this ape, and described under the name of the musculi interossei accessorii. The true position of the flexor brevis digitorum has been dealt with, the conclusion formed tends to confirm that originally laid down in the Challenger Reports but not that interpretation which is depicted in Quain's Anatomy. In the lower limb attention may be directed to the description and comparative anatomy of the following muscles: - the obturator internus, obturator externus, the adductor group, the flexors of the toes, tibialis anticus, flexor brevis digitorum, and musculus accessorius. In the trunk muscles special attention was directed to the dissection of the sheath of the rectus and I was able in an unmistakeable manner to demonstrate the true condition of the parts. In the arterial supply of the limbs, the large artery running down the inner side of the tibia was the most important point noted, I have attempted to show its connection with the superficial division of the anastomotic artery. The veins of both limbs were peculiar. With regard to the nervous system the limb plexuses were dissected, the likeness born by the brachial to the sacral plexus is apparent in the diagrams. Throughout; the dissection has been opposed to the theory formed by Ruge as to the relationship between the muscles and nerves. Ruge holding that the muscles are to be looked upon as being the end organs of the nerves which supply them; this view being, he considers, infallible. The facts here shown demonstrate that if the muscles are to looked upon as the end-organs of any structure, and I think they are, then they must be looked upon as the end-organs of the motor cells in the anterior horn of grey matter in the spinal cord, from which their axons of supply are derived. The path by which these axons reach the muscle being immaterial. The distribution of the dorsal nerves tends to show that the intercostal :muscles are not to be regarded as being derived from from one myotome nor are the ribs and lineae transversae of the rectus abdominis to be looked upon as being strictly inter segmental structures. I have been able to corroborate Professor Hepburn's statement that in this animal the pronator quadratus muscle was supplied by the posterior inter-osseous nerve. This statement was followed up and the fibrils traced to the median nerve higher up the arm. In addition the significance of the communication between the median and ulnar nerves in the forearm of the apes was made out in connection with the deep supply of the muscles of the palm.