262 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



parts — i. e., of serial homology. In general terms as ap- 

 plied to the whole skeleton, it may be stated thus: As 

 the whole vertebrate kingdom is made up of a repeti- 

 tion of individuals constructed on the same plan, but 

 modified according to the place and function in the verte- 

 brate scale, so each individual vertebrate is made up of a 

 repetition of segments, similar in plan but modified ac- 

 cording to its place and function in the series of segments. 

 This is, of course, best brought out in the skeleton. 



Take any vertebrate, such as a fish or a man. Make 

 a cross section of the body and look at the end. What 

 we see is shown diagrammatically in Fig. 166. The sec- 

 tion of the skeleton consists 

 of three parts — centrum, c, an 

 arch above (neural arch, n) 

 surrounding nervous centers, 

 and an arch below (visceral 

 arch, v) surrounding the vis- 

 ceral cavity. The whole is 

 enveloped in muscle and skin, 

 with which we have no con- 

 cern now. The three parts 

 named above constitute one 

 segment of the skeletal axis, 

 and may be called a vertebra. 

 Now the whole skeletal axis 

 may be regarded as made up 

 of a repetition of such skeletal 

 segments or vertebrne modi- 

 fied according to place and function in the series (Fig. 

 167). The centrums repeated form the vertebral col- 

 umn, the neural arches repeated constitute the neural 

 canal, in which are lodged the nervous centers, and the 

 visceral arches repeated make the visceral cavity. But 

 these segments are modified according to the place in 



Fig. 166. — Cross section through 

 a fish : v, visceral system ; 

 c, vertebra ; b, blood system ; 

 «, nervous system. 



