A history of land mammals in the western hemisphere

by William Berryman Scott

The anterior end of the skull is formed by a pair of rather small bones, the premaxillaries, which carry the incisor teeth; they bound the sides of the nasal opening, or anterior nares, reaching to the nasals, when the latter are of ordinary length; they also form the front end of the hard or bony palate, which divides the nasal passage from the mouth. The maxillaries, or upper jaw-bones, make up nearly all of the facial region on each side and send inward to the median line from each side a bony plate which together constitute the greater part of the hard palate; the remainder of the upper teeth are implanted in the maxillaries. A varying proportion of the hinder part of the hard palate is formed by the palatines, which also enclose the posterior nares, the opening by which the nasal passage enters the back part of the mouth. The maxillary of each side extends back to the orbit, which it bounds anteriorly and in the antero-superior border of which is the usually small lachrymal. The inferior, and more or less of the anterior, border of the orbit is made by the cheek-bone (malar or jugal) which may or may not have a postorbital process extending up toward that of the frontal; when the two processes meet, the orbit is completely encircled by bone, but only in monkeys, apes and Man is there a bony plate given off from the inner side of the postorbital processes, which extends to the cranial wall and converts the orbit into a funnel-shaped cavity. For most of its length, the jugal projects freely outward from the side of the skull and extends posteriorly beneath a similar bar of bone, the zygomatic process of the squamosal. This process and the jugal together constitute the zygomatic arch, which on each side of the skull stands out more or less boldly, and the size and thickness of which are subject to great variation in different mammals, the massiveness of the arch being proportional to the power of the jaws. One of the principal muscles of mastication (the masseter) is attached to the zygomatic arch.

The squamosal itself is a large plate, which makes up a great part of the side-wall of the cranium and articulates above with the frontal and parietal; it also supports the lower jaw, the articular surface for which is called the glenoid cavity. The lower jaw is held in place by the postglenoid process, which is a projection, usually a transverse ridge, behind the cavity. Back of the postglenoid process is the entrance to the middle ear, the auditory meatus, which may be merely an irregular hole, or a more or less elongated tube. The meatus is an opening into the tympanic, a bone which at birth is a mere ring and in some mammals remains permanently in that condition, but as a rule develops into a swollen, olive-shaped auditory bulla, which sometimes reaches enormous proportions, especially in nocturnal mammals. The labyrinth of the internal ear is contained in the periotic, a very dense bone which is concealed in the interior of the cranium, but in many mammals a portion of it, the mastoid, is exposed on the surface between the squamosal and occipital.

The lower jaw-bone (inferior maxillary, or mandible) is the only freely movable element of the skull; it consists of two halves which meet anteriorly at the chin in a contact of greater or less length, called the symphysis. In nearly all young mammals and in many adult forms the two halves of the lower jaw are separate and are held together at the symphysis only by ligaments, while in others, as in Man, they are indistinguishably fused to form a single bone. Each half consists of two portions, a horizontal part or ramus and an ascending ramus or vertical part; the former supports all of the lower teeth, and its length, depth and thickness are very largely conditioned by the number and size of those teeth. The ascending ramus is a broad, rather thin plate, divided at the upper end into two portions, the hinder one of which terminates in the condyle, a rounded, usually semicylindrical projection, which fits into the glenoid cavity of the squamosal. The anterior portion of the ascending ramus ends above in the coronoid process, which serves for the insertion of the temporal muscle, the upper portion of which is attached to the walls of the cranium and thus, when the muscle is contracted, the jaws are firmly closed; the coronoid process passes inside of the zygomatic arch. The lower jaw is therefore a lever of the third order, in which the power is applied between the weight (i.e. the food, the resistance of which is to be overcome) and the fulcrum, which is the condyle. At the postero-inferior end of the ascending ramus is the angle, the form of which is characteristically modified in the various mammalian orders and is thus employed for purposes of classification.

The hyoid arch is a U-shaped series of small and slender bones, with an unpaired element closing the arch below; each vertical arm of the U is attached to the tympanic of its own side and the whole forms a flexible support for the tongue, but with no freely movable joint like that between the lower jaw and the squamosal.

The mammalian skull in its primitive form may be thought of as a tube divided into two parts, of which the hinder one is the brain-chamber, or cranial cavity, and the forward one the nasal chamber or passage. With the growth of the brain and consequent enlargement of the cranium, this tubular character is lost; and various modifications of the teeth, jaws and facial region, the development of horns and tusks, bring about the many changes which the skull has undergone.

This brief sketch of the skull-structure is very incomplete, several of its elements having been altogether omitted and only those parts described which are needful in working out the history and descent of the various mammalian groups.

The second portion of the axial skeleton is the backbone, or vertebral column, which is made up of a number of separate bones called vertebræ. These are so articulated together as to permit the necessary amount of flexibility and yet retain the indispensable degree of strength. The function of the backbone is a twofold one: (1) to afford a firm support to the body and give points of attachment to the limbs, and (2) to carry the spinal cord, or great central axis of the nervous system, in such a manner that it shall be protected against injury, a matter of absolutely vital necessity.

While the vertebræ differ greatly in form and appearance in the various regions of the neck, body and tail, in adaptation to the various degrees of mobility and strength which are required of them, yet they are all constituted upon the same easily recognizable plan. The principal mass of bone in each vertebra is the body, or centrum, which is typically a cylinder, or modification of that form, and the two ends of the cylinder are the faces, by which the successive vertebræ are in contact with one another. In the living animal, however, the successive centra are not in actual contact, but are separated by disks of cartilage (gristle) which greatly add to the elasticity of the column. From the upper surface of the centrum arises an arch of bone, the neural arch, enclosing with the centrum the neural canal, through which runs the spinal cord. As already mentioned, the protection of the spinal cord is essential to the life of the animal, yet this protection must be combined with a certain flexibility, both lateral and vertical. Mere contact of the centra, even though these be held in place by ligaments, would not give the column strength to endure, without dislocation, the great muscular stresses which are put upon it. Additional means of articulation between the successive vertebræ are therefore provided, and these vary in size, form and position in different regions of the backbone, in nice adjustment to the amount of motion and degree of strength needed at any particular part of the column. Of these additional means of articulation, which are called the zygapophyses, each vertebra has two pairs, an anterior and a posterior pair, placed upon the neural arch. From the summit of the arch arises the neural spine, a more or less nearly straight rod or plate of bone, which may be enormously long or extremely short, massive or slender, in accordance with the muscular attachments which must be provided for. Finally, should be mentioned the transverse processes, rod-like or plate-like projections of bone, which arise, one on each side of the vertebra, usually from the centrum, less commonly from the neural arch; these also differ greatly in form and size in the various regions of the column. Anatomists distinguish several other processes of the vertebra, but for our purpose it is not necessary to take these into consideration.

Fig. 10.—First dorsal vertebra of Wolf from the front. cn., centrum. r., facet for the head of the rib. r′., facet for the tubercle of the rib. tr., transverse process. pr.z., anterior zygapophyses. n.sp., neural spine.