This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
For many of the American monkeys a fifth hand has been provided, formed by their long and muscular tail, which, from its extreme flexibility, can be forcibly twisted around any foreign object, and holds it with a tenacious grasp. Thus abundantly furnished with prehensile instruments, the Quadrumana are obviously most excellent and accomplished climbers, springing fearlessly through the forest by strong and vigorous leaps, or chasing their prey even to the topmost branches of the trees wherein they live.
(2232). But however grotesquely some of the more anthropoid Quadrumana resemble the human race, the approximation, even in their outward form, is at best exceedingly remote. The lower tribes, such as the Lemurs of Madagascar, walk on all-fours like cats, and are still remarkable for their long and fox-like muzzle. The brutal and ferocious Baboons are scarcely more human in their appearance; and even in the most elevated species, called by the vulgar "Wild men of the woods," the interval that separates them from humanity is wide indeed!
(2233). Taking the skeleton of the Orang-Outang (Simla Satyrus) as one of the most perfect examples met with in the class under consideration, it is at once evident that such an animal is by no means adapted to walk in an erect position, although well fitted to maintain a semi-upright attitude, such as is best calculated for climbing. The skull, whose very outline indicates brutal ferocity, is armed with canine teeth scarcely less formidable than those of the Tiger; and the massive jaws of this creature are moved by muscles almost equally powerful. It is true that the protuberance of the face is considerably diminished, and the facial angle thus materially enlarged; but to make up for the feebleness of the upper jaw, consequent upon this reduced size of the bones composing it, additional strength is needed to resist the strong pressure of the enormous temporal muscles. This is given by adding strong buttresses to the outer angle of the orbit, formed by the union of the frontal and the jugal bones, and thus the whole outline of the face becomes more humanized.
(2234). Another advance towards the condition of the human skull is apparent in the position of the foramen magnum, and of the condyles of the occipital bone, which are considerably advanced forwards beneath the base of the cranium, thus allowing the head to be articulated to the atlas at a very considerable angle with a line drawn through the axis of the spine, - a condition evidently favourable to the erect posture.
(2235). The thorax is well formed and capacious, giving great freedom of respiration: but the spinal column is short and clumsy; neither does it present those graceful sigmoid curves that convert the human spine into a perfect spring, upon the top of which the head is carried.
(2236). The arms are of inordinate length and extremely powerful, the joints perfect, and the clavicle well formed. But in the construction of the pelvic extremities the differences between this and the human skeleton become strikingly apparent. The pelvis is long, and the ossa ilii narrow; the thighs and legs so short, that, when the creature stands erect, the tips of the fingers almost touch the ground. The protuberance of the os calcis is very slight; and thus the posterior hands, although well adapted for taking hold of any object, are but ill calculated to sustain the weight of the body in an upright posture. Upon the ground, indeed, the living animal puts the spectator in mind of a human being crippled in the lower extremities; but in its native trees, these members, like those of the Sloth, are admirably suited to the circumstances under which the Orang is ordained to live.
Fig. 395. Skeleton of Orang-Outang.
(2237). Having thus introduced the reader to the different orders of Mammalia, as well as to the principal differences observable in the arrangement of their osseous system, we must briefly glance at some few points connected with their myology, selecting those that seem most worthy of being specially pointed out to the notice of the anatomical student.
(2238). To enumerate all the varieties that occur in the disposition of the muscular system in vertebrate animals would, of course, be incompatible with the extent of this work; and perhaps, even were it practicable, the details would scarcely possess much interest to the beginner in comparative anatomy. Considered generally, indeed, the muscular system of quadrupeds conforms very accurately in its arrangement to that of the human subject; and for the most part the same names are applicable to the individual muscles, allowance being made for such modifications in the manner of their origins and insertions as are rendered necessary by the disposition of the skeleton, or in order to accommodate them to the performance of special functions. To enumerate, therefore, the muscles of the jaws, of the neck, of the spine, of the chest, of the abdomen, or even of the extremities, in such genera as have the members last mentioned completely developed, would only be to repeat circumstances with which the human anatomist is already familiar; nevertheless there are some points of practical importance connected with this part of our subject that must not be altogether passed over in silence.
(2239). The diaphragm is a muscle only met with in the class before us, and in all Mammalia it forms the great agent in respiration, dividing the thoracic from the abdominal cavity by a broad musculo-tendinous septum, and presenting a disposition in all essential particulars similar to that of Man.
(2240). Another muscle of considerable anatomical interest is the cutaneous muscle provided for the movements of the integument. In many tribes, more especially those which, like the Hedgehog, the Echidna, and the Porcupine, have the skin covered with spines, this muscle is extremely developed, investing the greater part of the body with a thick layer of muscular fibres, called not improperly the panniculus carnosus. In Man, too, this muscle exists, but under a very different aspect, being only found in certain regions of the body, where it forms numerous cutaneous muscles adapted to different offices. In the neck, where it is principally developed, it is called the platysma myoides: in the facial region it is likewise of great importance, - the occipito-frontalis, the corrugator supercilii, and other muscles connected with the expression of the countenance, being indubitably but portions of the fleshy pannicle. In the palm of the hand it is slightly visible, forming the palmaris brevis; and even the little muscles connected with the external ear may be referred to the same series.