THE TELENCEPHALON. 1151 



it bends, the callosal gyrus is markedly reduced in width by the encroachment of the 

 calcarine fissure, the narrowed tapering tract thus formed being the upper part of the 

 isthmus (isthmus gyri fornicati), which below joins the similarly reduced upper end 

 of the hippocampal convolution and so establishes the continuity between the two 

 parts of the lobe. 



The hippocampal gyrus (gyrus hippocampi) curves forward from the isthmus 

 along the mesial border of the tentorial surface of the hemisphere towards the apex 

 of the temporal lobe, which, however, it fails to reach (Fig. 992). Its anterior 

 extremity is distinctly thickened and forms a rounded hook-like projection, the 

 uncus, which is recurved and directed backward and inward. The uncus is 

 separated from the apex of the temporal lobe by the incisura temporalis (fissura 

 rhinica), whilst the hippocampal convolution is marked off laterally by the anterior 

 part of the collateral fissure. Although blended with the gyrus hippocampi and 

 seemingly a part of the limbic lobe, the uncus, strictly considered, belongs to the 

 rhinencephalon and not to the limbic lobe (Turner, Elliot Smith). The posterior 

 end of the hippocampal convolution is incised by the anterior extremity of the 

 calcarine fissure and so divided into two parts ; of these the upper aids in forming 

 the isthmus and is continuous with the callosal gyrus, whilst the lower one blends 

 with the front part of the gyrus lingualis of the occipital lobe. 



The Rhinencephalon. — Although a division of fundamental importance and 

 differentiated at a very early period in the development of the human telencephalon, 

 in the brain of man it is represented by structures, which to a great extent are rudi- 

 mentary and feeble expressions of the bulky corresponding parts in the brains of 

 many of the lower animals. Its small size in man, as compared with the voluminous 

 structures seen in some mammals in which the rhinencephalon constitutes a large 

 part of the entire hemisphere, is no doubt associated with the relatively feeble olfac- 

 tory sense possessed by man. It is probable, however, that other and unknown 

 factors are responsible for the development of this part of the hemisphere to a degree 

 disproportionate to the olfactory capacity of the animal, as strikingly observed among 

 the lower vertebrates. The conclusions deduced from comparative studies empha- 

 size the fundamental character of the rhinencephalon as phylogenetically being the 

 oldest part of the hemisphere. Indeed of such primary morphological significance 

 is the rhinencephalon that it is termed the archipallhim, as distinguished from the 

 neopallium, which comprises almost the entire remainder of the hemisphere with the 

 exception of its nucleus, the corpus striatum. 



As seen in the human brain, the rhinencephalon includes the rudimentary olfac- 

 tory lobe — represented by the olfactory bulb, the olfactory tract with its roots, the 

 olfactory trigone, and the parolfactory area — and the uncus and a number of acces- 

 sory parts, including the anterior perforated space, the gyrus subcalloius, the sep- 

 tum lucidum, the fornix, the hippocampus and the gyrus dentatus. Some of these 

 accessory structures can be understood only after their relations to other parts of 

 the brain have been considered. Deferring the details of certain of these struc- 

 tures, as the septum lucidum, the fornix, and the hippocampus major, until the 

 lateral ventricles are described (page 1160), it will sufhce for the present to point 

 out their general features as related to the rhinencephalon. 



The Olfactory Lobe. — This division of the adult human brain is small and 

 rudimentary and comprises the olfactory bulb, the olfactory tract, the olfactory 

 trigone and the parolfactory area (Fig. 993). Of these all but the last lie on the 

 inferior surface of the brain, whilst the parolfactory area occupies a small space on 

 the mesial aspect of the hemisphere. 



The olfactory bulb (bulbus olfactorius) is an elongated irregularly oval swell- 

 ing, about 10 mm. long, from 3-4 mm. wide and about 2.5 mm. thick, which behind 

 is continuous with the olfactory tract and below receives the olfactory filaments. Its 

 upper surface underlies the olfactory sulcus of the orbital aspect of the frontal lobe, 

 and its under one rests upon the cribriform plate of the ethmoid bone, through the 

 apertures of which the bundles of the olfactory nerve-fibres ascend from the nasal 

 mucous membrane to the bulb. 



The structure of the olfactory bulb shares the general rudimentary condition which charac- 

 terizes the lobe in man, the bulb having lost the central cavity {ventricuUis bulbi olfactorii), 



