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.
(2241). In Whales, no pelvis or posterior extremities exist; it is needless, therefore, to remark that the whole of the muscular system appropriated to those parts in higher animals must be totally wanting; but, in return, the muscles connected with the caudal portion of the spine are amazingly powerful, so as to render the horizontally-expanded tail an instrument of propulsion adequate to the necessities of these unwieldy animals. A large triangular muscle is found in the Cetacea, apparently replacing the quadratus lumborum, the psoas, and the iliacus, which arises from the lower surface of the last rib, from the last dorsal vertebra, and also from those of the loins and sacrum: from this powerful assemblage of muscular fasciculi, tendons are given off, to be inserted into the lower surface of the bones that support the tail, converting this organ into a mighty oar, adapted by its position to bring the creature with all speed to the top of the ocean in search of air. It is, as might be supposed, in the muscles of the limbs that the most important differences exist.
In the anterior extremities, for example, the presence or absence of a clavicle will materially affect the disposition of the muscles of the shoulder, as will also the existence of a coracoid process to the scapula; nevertheless in their general arrangement they conform to those of Man. The rhomboid muscles, which to creatures walking on all-fours must be important agents, are generally found in quadrupeds to take their origin as far forward as the head: the serrati magni, likewise, whereby in the prone position the weight of the body is as it were suspended from the scapula, must be immensely strong.
(2242). The muscles acting upon the arm are similar in all the Mammalia; but in the fore-arm, as might be expected from the very variable condition of this part of the skeleton, the disposition of the muscular system varies too, and even the existence of many muscles could not be expected: thus as the movements of pronation and supination are, from the immoveable condition of the bones of the fore-arm, impracticable in the Cetaceans, the Ruminants, the Solipeds, and others, the pronators and supinators are denied; or, if their representatives exist, they become simply assistants in flexion and extension. The flexors and extensors of the wrist are pretty constant; but the muscles devoted to the hand and fingers vary in almost every order. The palmaris longus, although generally present where the hand is flexible, is wanting where its action upon the palmar fascia would be useless, as for example in the Ungulate tribes.
(2243). In quadrupeds there are two extensor tendons appropriated to each of the fingers that correspond to the four outer fingers of the human hand, whilst in Man the index and little fingers only have auxiliary extensors.
(2244). The abductor and extensor muscles of the thumb are not so perfectly developed in any animals as they are in the human hand. The short extensor, in fact, is wanting even in Monkeys; and in the lower orders of quadrupeds even the extensor longus and abductor are blended together, or totally wanting.
(2245). The deep and superficial flexors of the fingers are very generally met with, the number of tendons furnished by each corresponding of course to that of the fingers themselves; but in the Solipeds the two muscles are almost blended together. Even in the Ruminants, although these muscles remain separate, their tendons become confounded together, and divide again, to be inserted into the phalanges to which they are appropriated. In these Ungulata, too, as we need scarcely say, the lum-hricales and interossei are quite deficient; and the short muscles of the thumb are completely developed only in Man and in the Quadrumana.
(2246). It is in the human species only that the lower extremities are organized so as to maintain the body in the erect position; and in consequence, the glutcei muscles in the human body are enormously developed when compared with those of the lower animals; but the other muscles derived from the pelvis and thigh present but slight differences throughout the whole class under consideration. In the leg and foot likewise it is not difficult to identify the muscles that correspond to those found in the human subject, but, as in the anterior extremity, modified in their disposition and mode of insertion in accordance with the construction of the skeleton.
(2247). The articulations whereby the different pieces composing the Mammiferous skeleton are connected to each other are constructed upon the same principles as in the human body, insomuch that to describe them even in general terms would be useless.
(2248). The bones of the cranium and face, as in Man, are joined together by harmony or by suture. The articulations of the lower jaw are double, each presenting an interarticular cartilage, except in the Cetacea, where, instead of such a structure, a very thick, matted, ligamentous substance, having its interstices filled with oil, passes directly from the condyles of the jaw to the temporal bones.
(2249). The joints of the spine, thorax, and pelvis are all constructed upon the same principles as the corresponding articulations in the human subject; and the same may, with slight exceptions, be said of those of the extremities. The chief differences will be found in the connexion between the radius and ulna, the movements of rotation becoming gradually less manifest as we descend from Man: the tibia and fibula, too, ultimately become completely anchylosed to each other. The hip-joint contains an internal liyamentum teres; but in a few instances, e. g. the Ornithorhynchus, the Echidna, the Sloths, the Elephant, the Seals, and the Orang-Outang, this round ligament is deficient. The arrangement of the other articulations will be at once apparent on reference to the figures of the different skeletons already given.
(2250). Turning to the digestive system of Mammiferous animals, their teeth first invite our attention. We have already, when describing the osseous framework of these elevated beings, exposed their general arrangement in the jaws of the different orders; but it still remains for us to explain the varieties of their structure and the mode of their formation.