A history of land mammals in the western hemisphere

by William Berryman Scott

I. The Skeleton

I. The most obvious distinction of the skeletal parts is into axial and appendicular portions, the former comprising the skull, backbone or vertebral column, ribs and breast-bone or sternum, and the latter including the limb-girdles, limbs and feet. In the axial skeleton only the ribs and certain bones of the skull are paired, but in the appendicular all the bones are in pairs, for the right and left sides respectively.

Fig. 7.—Skull of Wolf (Canis occidentalis). P.Mx., premaxillary. Mx., maxillary. Na., nasal. L., lachrymal. Ma., malar or jugal. Fr., frontal. Pa., parietal. Sq., squamosal. Zyg., zygomatic process of squamosal. O.S., orbitosphenoid. Pl., palatine. M., mandible. cor., coronoid process of mandible. m.c., condyle of mandible. ang., angular process of mandible. p.g., postglenoid process of squamosal. Ty., tympanic (auditory bulla). mas., mastoid. p.oc., paroccipital process. con., occipital condyle. Ex.O., exoccipital. S.O., supraoccipital.

The skull is a highly complex structure, made up of many parts, most of which are immovably fixed together, and performing many functions of supreme importance. In the first place, it affords secure lodgement and protection for the brain and higher organs of sense, those of smell, sight and hearing, and second, it carries the teeth and, by its movable jaws, enables these to bite, to take in and masticate food. The portion of the skull which carries the brain, eyes and ears, is called the cranium, and the portion in front of this is the face, the boundary between the two being an oblique line drawn immediately in front of the eye-socket ([Fig. 7]). A great deal of the endless variety in the form of the skull of different mammals depends upon the differing proportions of cranium and face. In the human skull, for example, the cranium is enormously developed and forms a great dome, while the face is shortened almost to the limit of possibility; the skull of the Horse, on the other hand, goes to nearly the opposite extreme of elongation of the facial and shortening of the cranial region. The posterior surface of the skull, or occiput, is made up of four bones, which in most adult mammals are fused into a single occipital bone. At the base of the occiput is a large opening, the foramen magnum, through which the spinal cord passes to its junction with the brain; and on each side of the opening is a large, smooth, oval prominence, the occipital condyles, by means of which the skull is articulated with the neck. The paroccipital processes are bony styles of varying length, which are given off, one on each side external to the condyles. The boundary of the occiput is marked by a ridge, the occipital crest, which varies greatly in prominence, but is very well marked in the more primitive forms and tends to disappear in the more highly specialized ones. The roof and much of the sides of the cranium are formed by two pairs of large bones, the parietals behind and the frontals in advance; along the median line of the cranial roof, where the two parietals meet, is usually another ridge, the sagittal crest, which joins the occipital crest behind. The sagittal crest also varies greatly in prominence, being in some mammals very high and in others entirely absent, and, like the occipital crest, is a primitive character; as a rule, it is longest and highest in those mammals which have the smallest brain-capacity. As pointed out by Professor Leche, the development of the sagittal crest is conditioned by the relative proportions of the brain-case and the jaws. Powerful jaws and a small brain-case necessitate the presence of the crest, in order to provide sufficient surface of attachment for the temporal muscles, which are important in mastication, while with large brain-case and weak jaws the crest is superfluous. Though the brain-case proper may be quite small, yet it may have its surface enormously increased by great thickening of the cranial bones, as is true of elephants and rhinoceroses, and in them sufficient surface for attachment is afforded to the muscles without the development of a crest.

Fig. 8.—Skull of Wolf, top view. P.Mx., premaxillary. Na., nasal. Ma., malar or jugal. L., lachrymal. Fr., frontal. Sq., squamosal. Pa., parietal. S.O., supraoccipital.

Fig. 9.—Skull of Wolf, view of base. P.Mx., premaxillary. Mx., palatine process of maxillary. Pl., palatine. Fr., frontal. Pt., parietal. Ma., malar or jugal. Sq., glenoid cavity of squamosal. B.S., basisphenoid. B.O., basioccipital. Ty., tympanic (auditory bulla). p.oc., paroccipital process. con., occipital condyle. S.O., supraoccipital.

The structure of these cranial bones, more particularly of the parietals, is subject to important changes; in most mammals they are of moderate thickness and have dense layers, or “tables,” forming the outer and inner surfaces and, between these, a layer of spongy bone. In many large mammals, however, especially those which have heavy horns or tusks, the cranial bones become enormously thick and the spongy layer is converted into a series of communicating chambers, or sinuses, the partitions between which serve as braces, thus making the bone very strong in proportion to its weight. Sinuses are very generally present in the frontals and communicate by small openings with the nasal passage, even in genera of moderate size and without horns or tusks. The frontals form the roof of the eye-sockets, or orbits, and usually there is a projection from each frontal, which marks the hinder border of the orbit and is therefore called the postorbital process. The roof of the facial region is made by the nasals, which are commonly long and narrow bones, but vary greatly in form and proportions in different mammals; in those which have a proboscis, like tapirs and elephants, or a much inflated snout, such as the Moose (Alce) or the Saiga Antelope (Saiga tatarica) the nasals are always very much shortened and otherwise modified in form.