258 THE BRAIN OF QUADRUPEDS AND 



(fig. 72, c, c) and the diverging Fornix below, are two thin 

 vertical and almost parallel septa. These septa represent 

 the inner walls of the ' lateral ventricles ' and constitute 

 parts, therefore, of the contiguous inner faces of the two 

 Cerebral Lobes. They, together with the great trans- 

 verse commissure above and the Fornix below, form the 

 boundaries of a narrow and somewhat triangular cavity 

 known as the ' fifth ventricle.' This small ventricle is 



FIG. 72. Brain of Horse, longitudinal section through its centre, showing internal 

 surface of Cerebral Hemisphere. (Solly, after Leuret.) cc, Corpus callosum, between 

 it and the Fornix below and behind, is the ' fifth ventricle.' p, Thalamus ; co, the 

 middle or soft commissure ; t q, corpora quadrigemina, in front of which is the 

 Pineal body, with one of its ' peduncles ' passing forwards along the upper border of 

 the corresponding Thalarnus, and behind it the cut surface of the middle lobe of 

 the Cerebellum, e, Olfactory lobe ; o, olivary body. 



quite diifeient from, and also quite unconnected with, the 

 other four brain cavities, which are all of them continuous 

 with one another as are the corresponding antecedent 

 cavities met with in the early developmental phases of 

 the brain. But the ' fifth ventricle ' obviously could not 

 come into existence till the Corpus Callosum and Fornix 

 had become developed. Consequently no such cavity 

 exists in Birds, Reptiles, Amphibia, or Fishes.* 



* The arrangement of these central parts of the Brain in lower 

 Quadrupeds has been well described and figured by Prof. Flower 

 in the Philosoph. Trans, for 1865. 



