Identifier: zologytextbook00cock (find matches)
Title: Zöology; a textbook for colleges and universities
Year: 1920 (1920s)
Authors: Cockerell, Theodore D. A. (Theodore Dru Alison), 1866-1948
Subjects: Zoology
Publisher: Yonkers-on-Hudson, N. Y., World book company
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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in the mouth, we findthe teeth. It can be seen in the sharks that the teethare structures of the skin, not differing essentially fromthe spines which may be found on the outer surface ofthe animal. The number at first is very great, but asevolution proceeds they are reduced and specialized,and become firmly attached to the jaw bones. Ex-treme types of specialized teeth, like those of the ele-phant and the horse, seem to have little in common withthe simple conical structures of many fishes and reptiles.Many invertebrates possess teeth of different kinds, butthese are not homologous with those of vertebrates. The nervous 7. In all vertebrates there is a brain, serving as thechief controlling center of the nervous system, and there-fore of the whole body. The smaller nerve centers,called ganglia, are relatively very unimportant. Thebrain is continuous with the spinal cord, and both emita series of nerves which branch and extend to every part THE STRUCTURE OF THE VERTEBRATES 335 P ZD Mt
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From Zittds Palceontologie (after Claus) FIG. 123. Skeleton of an Egyptian vulture. Rh, cervical vertebrae; DM, thoracicvertebra; Cl, clavicle; Co, coracoid bone; Sc, scapula; Si, sternum; //, ilium;Is, ischium; Pb, pubis; H, humerus; R, radius; U, ulna; CC, carpus; Me, meta-carpus; p, p, p, phalanges of the three digits; Fe, femur; T, tibia; F, fibula;Tm, tarsometatarsus; Z, toes. 336 ZOOLOGY of the body. These nerves are different in function;afferent or sensory nerves convey impulses to the brainor cord ; efferent or motor nerves convey them in the re-verse direction, and are the means whereby muscularactivity is stimulated. Primitively the brain is a swelling at the anterior endof the spinal cord, and in the course of evolution it be-comes divided into three vesicles known as the fore-,mid-, and hind-brain. These vesicles are hollow, andthe cavities become variously modified. The fore-brain gives rise to the cerebral hemispheres, which inman occupy most of the surface of the brain

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