354 MANUAL OF ZOOLOGY sect. 



Through its substance from end to end runs a narrow canal, 

 the central canal. 



In the brain of the dogfish the most anterior portion is 

 a thick mass of nerve matter indistinctly divided into two 

 lateral portions by a shallow depression. This is the pro- 

 sencephalon of the fore-brain. A pair of lobes given off 

 from this in front are the olfactory lobes. The prosencepha- 

 lon with a narrow region, diencephalon or thalamencephalon, 

 behind it, constitute the fore-brain. Behind the fore-brain 

 a pair of oval lobes, the optic lobes, constitute the dorsal 

 portion of the mid-brain, which comprises, in addition, a 

 thick mass of longitudinal nerve-fibres, lying below, and 

 connecting the hind-brain with the fore-brain. An elon- 

 gated median mass, indistinctly divided into lobes, is the 

 cerebellum, the anterior portion of the hind-brain. The 

 posterior division of the hind-brain, — medulla oblongata, — 

 broad in front, tapers posteriorly where it passes into the 

 spinal cord. 



The central canal of the spinal cord expands in the me- 

 dulla oblongata into a wide shallow cavity, roofed over only 

 by a thin membrane ; this is known as the fourth ventricle. 

 From this runs forwards a narrow passage, the iter or aque- 

 duct of Sylvius, expanding in front in the thalamencephalon 

 into a laterally compressed cavity, the third ventricle. From 

 this are given off a pair of lateral ventricles, passing into 

 the prosencephalon, each giving off a prolongation into the 

 corresponding olfactory tube. 



The roof of the third ventricle is very thin ; it is pro- 

 duced into a slender process — the epiphysis ox pineal body. 

 Its side walls are formed of two masses, the optic thalami ; 

 its floor is produced into a hollow prolongation, the infun- 

 dibulum, to the end of which avascular body, the hypophysis 

 or pituitary body is applied. 



