Nervous System of Fishes 103 



two lateral portions known as the optic lobes, 

 they receive fibres coming from the animal's 

 retina. The lateral walls of the mid-brain (or 

 more correctly the diencephalon) are known as 

 the optic thalami, in the substance of which 

 some of the fibres of the optic nerve terminate 

 together with numerous fibres derived from the 

 animal's olfactory lobes, and tactile sensory 

 organs. 



The mid-brain is prolonged anteriorly into 

 the fore-brain, which in cartilaginous fishes 

 consists of paired lateral lobes known as the 

 cerebral hemispheres. These at the anterior 

 end are prolonged forward as slender cylinders 

 of nervous matter, terminating in the olfactory 

 bulbs, which receive fibres from the animal's 

 olfactory sensory organs. (Fig. 13.) The fore- 

 brain contains a cavity known as the third 

 ventricle, whose lateral walls are thickened so 

 as to form the two corpora striata; these, with 

 the optic thalami, constitute the stem of the 

 brain or its basal ganglia. 1 The nervous matter 



1 In a recently published article on the anatomy and physiology 

 of the corpus striatum, bearing specially on its motor functions, 

 Dr. S. A. Kinner Wilson remarks that, " from a consideration of 

 the anatomical data, obtained by experiments on apes, and known 

 to be in part, if not entirely, identical in man, it is clear the 



