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.
Fig. 386. Skeleton of Hippopotamus.
(2200). The most important differences observable between the various genera of Pachydermatous Mammalia are found in the structure of their feet, and in the number and disposition of their toes. In the Elephant there are five to each foot; but in the living state they are so encased in the callous skin which forms a sort of hoof to the foot of this monstrous animal, that they are scarcely perceptible externally. In the Hippopotamus, above delineated, there are four, and also in the Hog tribes; but in the latter the two middle toes are disproportionately large. The Rhinoceros has only three toes to each foot; and other varieties in this respect might easily be pointed out.
(2201). In the Solidungula, or Solipeds, regarded by Cuvier as a family belonging to the order last mentioned, we have a tribe of animals quite peculiar as relates to the construction of their locomotive extremities.
(2202). In the Horse, for example (a creature obviously formed to be an assistant to the human race), so completely has every other consideration been sacrificed in order to ensure the utmost possible strength and solidity in the structure of the foot, that all the toes appear externally to have been solidified into one bony mass, which, being encased in a single dense and horny hoof, is not only strong enough to support the weight of the quadruped, and to sustain the shock produced by its most active and vigorous leaps, but becomes abundantly efficient to carry additional burdens, or to draw heavy loads in the service of mankind.
(2203). In the anterior extremity of a Soliped (fig. 387) the shoulder consists only of the scapula, there being no clavicle to connect it with the sternum. The humerus is short and very strong: the radius and ulna are partially consolidated together, so that all movements of pronation and supination are impossible. The carpus is composed of seven short bones disposed in two rows. The metacarpus is a single bone (the cannon bone), which, from its length and size, is commonly called the "fore leg" of the horse, the carpo-meta-carpal articulation being looked upon as the "knee." Lastly, the foot consists of three great phalanges; whereof the proximal is named the "pastern," the second the "coronary," and the distal phalanx the "coffin bone." In the macerated skeleton, however, the vestiges of two other toes are visible; but they are merely rudiments resembling osseous splints attached to each side of the metacarpus or cannon bone.
(2204). In the posterior limbs of the Horse the same peculiarities are observable, in the construction both of the leg and foot.
(2205). The Ruminantia constitute another order of quadrupeds of very great importance to mankind, distinguished by their remarkable habit of chewing the cud; that is, of bringing up the food again from the stomach into the mouth, for the purpose of undergoing a second process of mastication. They all have well-developed incisor teeth in the lower jaw, but none in the upper. The patient and thirst-enduring Camel, the stately Giraffe, the Ox, the Sheep, the Goat, the nimble Antelope, and the fleet and elegant Stag are all examples of this extensive order; but it is the skeleton of the last-mentioned alone that we shall select for delineation (fig. 388).
Fig. 387. Fore leg of the Horse.
(2206). The most remarkable feature observable in the Ruminant order of quadrupeds is, that, with the exception of the Camel tribe and the Musk-deer, the males, and sometimes the females, are provided with two horns attached to the os frontis, appendages not met with in any other Vertebrata. In some, as the Giraffe, these horns consist merely of a bony protuberance developed from each frontal bone, which is coated with a hairy skin derived from the common integument of the head. In others, as in the Ox, Goat, Antelope, etc, the bony nucleus of the horn is covered over with a sheath of corneous matter, giving it a hard and smooth surface.
(2207). Both the above kinds of horns are persistent; but in the Deer tribe the defences of the head, which are large and branched, are deciduous, being formed every year from a vascular skin that covers them externally during the period of their growth, but shrivels up and dries when they are completed. These horns fall off after a certain time, to be renewed again the following season. The mode of their formation, however, will be examined in another place.
(2208). In consequence of the weight of the horns in such species as possess weapons of this description, the head is necessarily extremely heavy; and in genera where the horns are wanting or feebly developed, as in the Camel or Giraffe, such is the length of the neck, that, even with a disproportionately small head attached to the extremity of so long a lever, incessant and violent muscular exertion would be needed to sustain or to raise it from the ground. This difficulty is obviated by a very simple and elegant contrivance: a broad band of ligament, composed of the same elastic tissue as that composing the ligamenta subjlava of the human spine, is extended from the tips of the elongated spinous processes of the back, and sometimes even as far backwards as the lumbar and sacral regions. This ligament, strengthened by additions derived from most of the vertebral processes over which it passes, runs forward to be fixed anteriorly to the crest of the occipital bone, and to the most anterior of the cervical vertebrae.
The whole weight of the cranium and neck being therefore fully counterbalanced by the elasticity of this suspensory ligament, the muscles of the neck act with every possible advantage, and all the movements of the head are effected with the utmost grace and facility.