43 2 ZOOLOGY. 



443. Skeleton. Some of the more elementary facts concerning the 

 skeleton may be summarized as follows. The vertebrae unite by flat faces, 

 and the five regions of the vertebral column (see 341) have a fair 

 degree of constancy as to numbers. The neck, with a very few excep- 

 tions, has seven vertebrae, the length of the neck depending on the length 

 of the vertebrae and not on their number. The trunk vertebrae, made 

 up of the thoracic and lumbar, usually vary within the limits 19-23. The 

 caudal vertebras are most variable of all. The bones of the skull in the 

 adult have their edges closely united by means of sutures (a species of 

 close joint, which does not allow of motion). The lower jaw, the hyoid 

 bone, and the small bones of the ear are the only movable bones in the 

 mammalian skull. The lower jaw articulates directly with the cranium. 

 The quadrate, which in reptiles and birds serves to articulate the jaw 

 with the cranium, has apparently changed its position and given rise to 

 one of the small bones of the middle ear. 



The pectoral girdle and arm bones are always present, but in the 

 whales and sea-cows the posterior are lacking. The digits are typically 

 five in number. In many carnivores these may be reduced to four, 

 terminating in claws. In the hoofed forms the toes are often reduced 

 to four, two, or even one (the horse). In such cases rudiments of the' 

 remaining digits may occur in the form of splints. 



444. Teeth. The teeth are produced by the skin, and come 

 to be lodged in pits in the bones of the jaws. While differing 

 in shape, the teeth always possess the crown, the fang or root, 

 the neck and the pulp-cavity. The bulk of the tooth is dentine 

 deposited by the dermis. Over this is a layer of enamel formed 

 by the epidermis. The cavity is more or less filled with " pulp 

 tissue " which is supplied with nerves and blood vessels. Most 

 mammals have only two sets of teeth, a milk set which ap- 

 pears early and is lost and a permanent set which replaces the 

 former. In some cases, however, there is only one set, and in 

 a few (e. g., whales) no teeth appear above the surface of the 

 gums. 



In the porpoises, dolphins and similar forms the teeth are 

 numerous, simple, and very much alike, but in the majority 

 of mammals there are at least three types of teeth. In the 

 front of the upper jaw (on the premaxillary bones) are simple, 

 chisel-shaped teeth, the incisors; behind these (the first tooth 

 on the maxilla) is the canine tooth, usually pointed and adapted 

 for tearing ; posterior to the canines are the grinders or molars. 



