Chap. XVI.] SOME OTHER MAMMALS. 271 



anteriorly through the * foramen of Monro.* Behind, it 

 is continuous by means of a passage beneath the Corpora 

 Quadrigemina (fig. 72, t q) with the Fourth Ventricle. The 

 Third Ventricle is likewise continuous below with the 

 * infundibulum ' of the Pituitary Body. At its posterior 

 and upper boundary is the peculiar pyriform structure 

 known as the Pineal Body, which is attached by two long 

 peduncles to the upper and inner borders of the Thalami 

 (fig. 72). This body itself lies against and just in front 

 of the Corpora Quadrigemina; it is, in proportion to 

 other parts, decidedly smaller than the corresponding 

 structures in Keptiles or Birds. It is extremely small in 

 the Eabbit and some other Eodents. 



The distinct Commissures seen in or in connection with 

 the Lateral and Third Ventricles are five in number. Of 

 these, three are to be found also (though in a very 

 rudimentary condition) in some of the lower Vertebrates, 

 while the two others appear for the first time in Quadru- 

 peds. 



The Anterior Commissure is a band of fibres of vari- 

 able thickness, which stretches across the anterior and 

 upper boundary of the Third Ventricle (fig. 87, 10), and 

 penetrates deeply through each Corpus Striatum to certain 

 surface regions of the Cerebral Hemispheres. It is larger 

 in Marsupials and Monotremes than in any other Mam- 

 mals, and in higher representatives of the class it is 

 usually thickest in those animals which have well-developed 

 Olfactory Lobes, since it seems to be a commissure serving 

 principally to bring the two cerebral centres of the sense 

 of Smell into relation with one another. In part it con- 

 nects the Olfactory Peduncles with one another, an din part 

 it serves to bring into relation those regions of the brain in 

 each hemisphere in and about the Hippocampi, to which 



