THE NERVOUS" SYSTEM. 271 



anterior ends alone being present. A small pocket- 

 like sac in the suspensory ligament of the liver, 

 between it and the pericardium, represents the ab- 

 dominal opening of the oviducts of the female. 

 From this sac the ducts can be traced a short distance 

 round the sides of the oesophagus. 



10. The claspers (see p. 220). 



11. The spermatozoa are delicate thread-like structures, 



each with a thickened head round which a spiral 

 membrane runs. 



VIII. DISSECTION OF THE NERVOUS SYSTEM. 



For this purpose a dog-fish should be taken which has been 

 hardened in spirit, the roof of the skull having been previously 

 removed so as to allow the spirit free access to the brain. The 

 specimen must be moistened from time to time to prevent the 

 nerves drying up, or else should be dissected in water or weak 

 spirit. 



The nervous system consists of (1) a central portion, the 

 brain and spinal cord, lodged within the skull and vertebral 

 column respectively ; and (2) a peripheral portion, the nerves 

 themselves, which connect the brain and spinal cord with 

 all parts of the body. 



From the mode of its development the central nervous 

 system is tubular, and it retains this character throughout 

 life. The central canal is very small in the spinal cord, but 

 widens out considerably in the brain, its local dilatations 

 being spoken of as the ventricles of the brain. 



A. The Dorsal Surface of the Brain. 



Remove the skin from the dorsal surface of the head. Dissect 

 away the eyelids on the right side so as to open up the orbit. 

 Slice away the roof of the cranium so as to expose the brain fully, 

 slicing through the auditory capsule on the right side, but leaving 

 it uninjured on the left. 



