NERVOUS SYSTEM 239 



8. The cloaca is the shallow depression between the pelvic 



fins into which, in the male, the rectum opens in front 

 and the urino-genital sinus behind. 



9. The pronephric or Miillerian ducts are rudimentary in 



the male, their anterior ends alone being present. 

 A small pocket-like sac in the suspensory ligament of 

 the liver, between it and the pericardium, represents 

 the abdominal opening of the oviducts of the female. 

 From this sac the ducts can be traced a short dis- 

 tance round the sides of the oesophagus. 



VIII. DISSECTION OF THE NERVOUS SYSTEM. 



For this purpose a dog-fish should be taken that 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 run from the brain and spinal cord to 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 ofvery small size in the spinal cord, 

 but widens ont'comiiideTSbry in the brain, its local dilatations 

 being spoken of as the ventricles of the brain. 



A. The Dorsal Surface of the Brain. 



Remove fhe 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, which lies 

 immediately behind the orbit, on the right side, but leaving 

 it uninjured on the left. 



