THE NERVOUS SYSTEM. 263 



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. 

 Prom this sac the ducts can be traced a short 

 distance round the sides of the oesophagus. 



10. The ciaspers (see p. 21'2). 



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 he taken which has been 

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

 removed so as to alloiv 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 luater 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 Lorsal Surface of the Brain. 



Bemove 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 tlie 

 brain fully, slicing through the auditory capsule on the right 

 side, but leaving it uninjured on the left. 



