CHAPTER XVII 



THE RHINENCEPHALON 



THE olfactory portions of the cerebral hemisphere may all be grouped to- 

 gether under the name rhinencephalon. Phylogenetically very old, this part of 

 the brain varies greatly in relative importance in the different classes of verte- 

 brates. The central connections of the olfactory nerves form all or almost all of 

 the cerebral hemispheres in the selachian brain (Fig. 13); while in the mammal 

 the non-olfactory cortex or neopallium has become the dominant part. Even 

 among the mammals there is great variation in the importance and relative 

 size of the olfactory apparatus. The rodents, for example, depend to a great 

 extent on the sense of smell in their search for food, and possess a highly developed 

 rhinencephalon. Such mammals are classed as macrosmatic. Man, on the 

 other hand, belongs in this respect with the microsmatic mammals, because in 

 his activities the sense of smell has ceased to play a very important part, and 

 his olfactory centers have undergone retrogressive changes. The carnivora and 

 ruminants are in an intermediate group. The sheep's brain furnishes a good 

 illustration of this intermediate type, and displays much more clearly than the 

 human brain the various parts of the rhinencephalon and their relation to each 

 other. 



Parts Seen on the Basal Surface of the Brain. A comparison of the basal 

 surface of the sheep's brain with that of the human fetus of the fifth month shows 

 a remarkable similarity in the parts under consideration (Figs. 197, 198). The 

 olfactory bulb, which is the olfactory center of the first order, is oval in shape and 

 attached to the hemisphere rostral to the anterior perforated substance. It 

 lies between the orbital surface of the cerebral hemisphere and the cribriform 

 plate of the ethmoid bone. Through the openings in this plate numerous fine 

 filaments, the olfactory nerves, reach the bulb from the olfactory mucous mem- 

 brane. It contains a cavity, the rhinoccele, continuous with the lateral ventricle 

 (Fig. 182). In the adult human brain the cavity is obliterated and the connec- 

 tion between bulb and hemisphere is drawn out into the long olfactory tract. 

 This is lodged in the olfactory sulcus on the orbital surface of the frontal lobe 

 and in transverse section presents a triangular outline (Fig. 172). It contains 



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