NERVOUS SYSTEM. VKKTKI'.KATA. 467 



exposed), is much the most serviceable of all the avenues of information 

 to the lowly mammal leading a terrestrial life and therefore becomes 



predominant ; and its particular domain the forebrain become* tho 



ruling portion of the nervous system. 



This early predominance of the sense of smell persists in most 

 mammals (unless an aquatic mode of life interferes and deposes it : 

 compare the Cetacea, Sirenia, and Pinnipedia for example) even though 

 a large neopallium develops to receive visual, auditory, tactile, and 

 other impressions pouring into the forebrain. In the Anthropoideaalone 

 of non-aquatic mammals the olfactory regions undergo an absolute (and 

 not only relative, as in the Carnivora and Ungulata) dwindling, which 

 is equally shared by the human brain, in common with those of the 

 other SimiidaB, the Cercopithecidce, and the Cebida3. But all the parts 

 of the rhinencephalon, which are so distinct in macrosmatic mammals, 

 can also be recognised in the human brain. The small ellipsoidal 

 olfactory bulb is moored, so to speak, on the cribriform plate of tho 

 ethmoid bone by the olfactory nerves so that, as the place of attachment 

 of the olfactory peduncle to the expanding cerebral hemisphere becomes 

 removed (as a result of the forward extension of the hemisphere) 

 progressively farther and farther backward, the peduncle becomes 

 greatly stretched and elongated. And as this stretching involves the 

 grey matter without lessening the number of nerve-fibres in the 

 olfactory tract, the peduncle becomes practically what it is usually 

 called, i. e. the olfactory " tract." The tuberculum olfactorium becomes 

 greatly reduced and at the same time flattened, so that it is not easy to 

 draw a line of demarcation between it and the anterior perforated 

 space. The anterior rhinal fissure, which is present in the early 

 human foetus vanishes (almost, if not altogether) in the adult. Part 

 of the posterior rhinal fissure is always present as the " incisura 

 temporalis," and sometimes (D. 71 0), especially in some of the non- 

 European races, the whole of the posterior rhinal fissure is retained in 

 that typical form which we find in the Anthropoid Apes. When this 

 occurs we can easily recognise the caudal limits of the pyriform lobe, 

 which otherwise becomes confused with the neopallium. 



Tho hippocampal fissure is of a peculiarly consistent nature, and is 

 found in all mammalian brains from OrnitJiorhynchus to Homo. The 

 rhinal fissure is equally sui generis and almost as constant as the hippo- 

 campal. A few small mammals, such as Notary ctes, Chlami/dojrfionu, 

 Clirysochloris, and some small Chiroptera, have no rhinal fissure. 



Of the sulci perhaps the most constant is the calcarine, which is 

 found in the Marsupials (both Poly- and Diprotodont), in tho larger 

 Chiroptera, Galeopithecus, (but not in any true Insectivore, nor, strange 



2 H S 



