272 THE BRAIN OF QUADRUPEDS AND 



the majority of the root fibres of such tracts proceed. It 

 is a structure, therefore, much larger in the greater num- 

 ber of Quadrupeds than it is in Man. In some of the 

 Cetacea the ' anterior commissure ' is so small as to be 

 almost non-existent. 



The Middle Commissure is a short and rather thick 

 bridge of soft ganglionic matter, which passes across the 

 middle of the Third Ventricle (figs. 87, 13 ; 72, c o) from 

 one thalamus to the other, and therefore serves to connect 

 these two great ganglia. 



The Posterior Commissure is small, and composed of 

 white fibres. It passes immediately in front of the base 

 of the Pineal body, and its fibres are prolonged, on each 

 side, into the substance of the posterior part of the Tha- 

 larnus. 



We come now to the commissures met with only in the 

 brain of Mammals. 



The Fornix is a double commissure, each half of which 

 suffices to connect two regions of the same Hemisphere 

 with one another viz., the Hippocampal region with the 

 inner part of the corresponding Thalamus. The two 

 halves of this structure come into contact only during a 

 small part of their course about the middle of it but 

 they are also brought into some sort of relation posterior 

 to this point by means of a stratum of cross fibres, the 

 nature and connections of which are described below. 



Along the inner side of the Hippocampus, as it projects 

 into the descending prolongation of the Lateral Ventricle, 

 a ridge or band of white fibres (' tenia hippocampi ') may be 

 traced upwards on each side (fig. 86, i), which soon becomes 

 free as the ' posterior pillar ' of the Fornix, and bends for- 

 wards and inwards over the Thalamus so as to join its 

 fellow on the opposite side, as above stated. Posterior to the 

 point of contact of these ' pillars ' with one another, certain 



