THE DOGFISH 27 



that has been preserved in brine should be taken, if possible, as 

 formalin renders the tissues hard and tough. 



The Axial Skeleton. The vertebral column consists of a suc- 

 cession of vertebrae closely joined together by ligaments; they 

 are also connected by the notochord,— a soft, pulpy mass which 

 runs through the axis of the vertebral column, filling the spaces 

 between the vertebrae. 



Each vertebra is made up of a cylindrical, deeply biconcave 

 body called the centrum, to which are attached dorsal and either 

 lateral or ventral projections. The concavities at the ends of the 

 centrum are joined by a central canal, and this hourglass-like space 

 is filled with the notochord. The inner surface of the centrum 

 is calcified. 



The dorsal projections of the centra form the neural arches; 

 they inclose the neural canal, within which lies the spinal cord. 

 Each* arch consists of a pair of neural processes, which form its 

 sides, and two small median neural spines, which form its roof. 

 Between the neural processes are the intercalary processes, which 

 are similar to them in shape and size. 



The lateral and ventral projections of the centrum are respec- 

 tively the transverse processes and the haemal processes, which are 

 the equivalents of each other. In the region of the trunk each 

 centrum bears a pair of transverse processes ; these are lateral or 

 lateroventral projections, at the distal end of each of which is 

 a short rib. In the caudal region, however, the ribs are absent and 

 the processes project ventrally and meet in the median plane, form- 

 ing the haemal arches. Within these arches lie the caudal artery 

 and vein. 



Exercise 32. Draw the following views : ( 1 ) a side view of several 

 consecutive vertebrae ; ( 2 ) an end view of a trunk vertebra with the 

 ribs belonging to it; (3) an end view of a caudal vertebra. 



Exercise 33. Split a portion of the spinal column by a sagittal incision 

 and draw the surface exposed. 



The skull is made up of two very distinct portions : the cranium, 

 within which is the brain, and the sides of which are formed by 

 the capsules containing the special sense organs ; and the visceral 



