180 ZOOLOGY. 



cavity, alternately large and small, runs through the wholo 

 vertebral column from end to end, and this is occupied by 

 a soft, gelatinous tissue the persistent remains of that 

 important embryonic structure the Notochord (chap. VI, 

 15). The variation in diameter of the notochord is shown 

 in figs. 88, A and 89 (cut through the middle of a centrum) 

 and 88, B (through the end of one). This cavity must not 

 be confused with the neural cavity containing the spinal 

 cord, which here (as in all cases) is altogether dorsal to the 

 centra of the vertebrae. From each centrum there extend 

 up a pair of neural plates to form the sides of the neural 

 arch, but these are shorter than the centra, and a series of 

 intervertebral neural plates, distinct from the centra, fill up 

 the gaps. A double series of neural spines, alternating 

 with the plates, completes the arch dorsally (fig. 93). From 

 the centra there come off ventro-laterally short transverse 

 processes, to which short cartilaginous ribs are articulated. 

 There are no zygapophyses, and the centra and neural 

 plates are connected by fibrous tissue, so that but little 

 motion is possible at any single point, though the column 

 as a whole is sufficiently flexible. 



As we pass back from trunk to tail, we find the ribs 

 cease, the transverse processes turn ventralwards, and finally 

 unite in a hcemal arch (enclosing the caudal artery and 

 vein), which may be distinguished from the neural arch 

 above by the gaps between the successive processes (fig. 94). 



Vent rally it is completed by a 

 series of haemal spines. The 

 only other difference of the 

 tail- from the trunk-vertebrse, 

 is that the neural spines are 

 less regular, there being often 

 three, instead of four, of them 



Fig. 94. MEDIAN LONGITUDINAL f (O 

 SECTION OF THREE CAUDAL VERTEBRA 1/O 



OF DOGFISH. W here the median nns 



come, there are cartilaginous 



fin-rays developed in continuation of the neural spines ; and 

 beyond these, in the fin itself, horny rays. 



1 2. The Skull (fig. 93) consists of a cartilaginous 



