158 PHYSIOLOGICAL SERIES. 



This is one of the simplest and most generalised types of 

 the mammalian brain, and presents a marked contrast 

 to the specialised Monotreme organ. 



Its most obtrusive feature is the relatively enormous si/e 

 of the olfactory bulbs, which are attached by short thick 

 peduncles to the front of the cerebral hemispheres. In 

 the great majority of mammals the olfactory apparatus 

 is largely developed ; and in the case of a terrestrial, offal- 

 eat ing animal, like Sarcopliilus, the importance of tlu 1 

 sense of smell becomes enormously enhanced and it becomes 

 the dominant sense. This finds expression (as in Perameles, 

 ride fig. 52) in the huge development of the olfactory- 

 bulb, in the large elliptical olfactory tubercle on the base of 

 the brain, in a pyriform lobe which forms a large part of the 

 ventral surface and almost half of the lateral aspect of the 



Fig. 45. (Nat. size.) 



OLF -- OLF TUBER 



PYfLL. 



hemisphere, and in a large hippocampal formation which 

 forms a considerable part of the mesial wall. 



This brain shows very clearly the definite connection- 

 which the olfactory peduncle establishes with both tin- 

 lateral and mesial walls of the cerebral hemisphere. UJH.M 

 the lateral aspect (fig. 45) the peduncle is direct I v 

 continued into the pyriform lobe, and a layer of medullary 

 fibres upon the latter becomes collected into a definite 

 bundle -the olfactory tract or so-called "external oll'a. 

 root" which proceeds backwards in the -hallow OT 

 between the pyriform lobe and the olfactory tubercle, 

 distributing fibres over the surfaces of both. The tract 

 finally ends in a little nodule behind the olfactory tubercle 

 (fig. 45, *) , which may be called the tubercle of the olfactory 

 tract [Ketzius calls it the " gyrus intermedius "]. 



