28 BULLETIN OF THE LABORATORIES 



the brain cavity as to cause the projecting olfactory lobes to be re- 

 flexed. The olfactory bulb is about 6 mm. in diameter, narrowing 

 posteriorly into the cms, and is thickly covered on the whole cephalic 

 and ventral surface with the roots of the olfactory nerves. 



The Prosencephalon. The general form is hemispherical, each 

 cerebral lobe being flattened on the median and ventral aspects. The 

 great development of the front part of the cerebrum is the most strik- 

 ing feature of the brain. The ventral surface is unusually flat and 

 broad. The pyriform and postrhinal lobes are not high, giving this 

 surface a very different appearance from that of the brains of the rab- 

 bit or Guinea-pig. 



The dorsal surface of the hemispheres is unconvoluted ; on their 

 ventral surface the rhinalis and postrhinalis form a continuous and 

 strong fissure running their whole length. The compression in the 

 region of the Sylvian fissure so conspicuous in most rodents is here 

 reduced to a minimum. No trace of the Sylvian fissure itself is no- 

 ticeable on the dorsal aspect ; it is however, described and figured by 

 Owen* in the case of Hystrix cristata as well marked, though short. 

 But there is on the ventral surface a shallow, though well defined, fis- 

 sure running from the chiasm laterad to the rhinalis, where it stops 

 abruptly. The pyriform lobe thus sharply defined is about 15 mm. 

 wide at the widest part opposite the hypophysis. Other small fissures 

 on the ventral aspect are occasionally present, but very variable. On 

 the median surface of the hemisphere the callosum is bounded on the 

 dorsal and caudal aspects by the so-called callosal fissure. Passing 

 ventrad from the splenium of the callosum the hippocampal fissure is 

 well marked. Caudad to the hippocampal fissure is another small fis- 

 sure passing caudo- dorsad to a point about as high as the splenium, 

 apparently a suggestion of the splenialis. On the olfactory cms are 

 two superficial tracts of. lighter color; the smaller and shorter passes 

 caudad and mesad, the other can be traced caudad and laterad as far 

 back as the chiasm. Their relations are discussed in the paragraph 

 on the internal topography of the rhinencephalon. The hippocampus 

 is essentially as in Geomys, and under that head this region will be 

 discussed, as the true relations are more easily traced in that type. 



^"Comparative Anatomy and Physiology of the Vertebrates," Vol. Ill, 

 n. no. 



