44 LECTURE III. 



losed to the under part of the centrum, to which part they are 

 moveably articulated in the tails of most reptiles and mammals ; space 

 being needed only for the protection of the ca- 

 rotids in the one case, and for the caudal artery 

 and vein in the other. In the chest, where the 

 central organ of circulation is to be lodged, an 

 expansion of the haemal arch takes place, analo- 

 gous to that which the neural arches of the 

 cranial vertebrse present for the lodgment of the 

 brain. Accordingly in the thorax, the pleura- 

 f?om ^thVneck'Lu PeUctn pophyscs {fig. 6. pX) are much elongated, and 

 the haemapophyses {fig. 6. A) are removed from 

 the centrum, and are articulated to the distant ends of the pleura- 

 pophyses ; the bony hoop being completed by the intercalation of 

 the heemal spine {fig. 6. hs) between the ends of the h^mapo- 

 physes. And this spine is here sometimes as widely expanded (in 

 the thorax of Birds and Chelonians, for example) as is the neural 

 spine (parietal bone or bones) of the middle cranial vertebra in 

 Mammals. In both cases, also, it may be developed from two lateral 

 halves, and a bony intermuscular crust may be extended from the 

 mid-line, as in the skull of the Hycena, and the breast bone of the 

 Hawk. 



The vertebrsB of the trunk present essentially their most simple, 

 though often apparently the most complete, condition in Fishes, in 

 which class a typical vertebra can only be obtained from the head ; 

 in the rest of the column, the liEemapophyses, for example, are always 

 absent or unossified. It is by no means true that the several elements 

 of a vertebra are found most isolated and distinct in the lowest classes ; 

 the neurapophyses are commonly anchylosed to the centrum in fishes, 

 but commonly remain isolated and distinct in reptiles ; the hfemal 

 canal is formed by modified parapophyses in fishes, but by isolated 

 and distinct hajmapophyses co-existing with transverse pi'ocesses 

 in reptiles and mammals. 



The number of vertebrae, or at least of neural arches, is governed 

 by the number of segments of the cerebro-spinal axis. These 

 segments in the spinal chord are chiefly indicated by the pairs of 

 spinal nerves. In the brain, the centres are more definitely indi- 

 cated by the ganglionic form under which they first make their 

 appearance ; but here, by the superaddition of fasciculi of nerve- 

 fibres for the special functions of the brain, the origins of essentially 

 single nerves become separated, and the motor roots divided from the 

 sensitive, as we see in the nerves of the eyeball. Hence, the cranial 

 vertebra3 do not correspond with the number of seemingly distinct 



