844 



THE NERVOUS SYSTEM 



lobe. The fusiform gyrus is bounded laterally by the inferior temporal sulcus, 

 which sometimes is continuous by a lateral twig across the posterior end of this 

 gyrus with the collateral fissure. 



The rhinencephalon. The rhinencephalon or olfactory brain includes those 

 portions of the cerebral hemisphere which are chiefly concerned as the central con- 

 nections of the olfactory apparatus. Owing to the preponderant development of the 

 other divisions of the hemisphere, the parts comprising this division appear relatively 

 but feebly developed in the human brain. In most of the mammals the sense of 

 smell is relatively much more highly developed, and in many of the larger mammals 

 the parts comprising the rhinencephalon are of greater absolute size than in man, 

 though their cerebral hemispheres may be considerably smaller. In the human 

 firtus the parts of the rhinencephalon are relatively much more prominent than after 

 the completed differentiations into the adult condition. In the broader sense of the 

 term the rhinencephalon includes those parts of the hemisphere usually classed as 

 comprising two lobes, viz., the olfactory lobe and the limbic lobe. Neither of these 

 is a 'lobe' in the sense of comprising a definite segment of the hemisphere, as do the 

 other lobes, and therefore the rhinencephalon cannot be called a distinct lobe. It is 

 so strung out that by one or the other of its parts it is either in contact or continuity 

 with each of the other lobes of the hemisphere. 



FIG. 627. BRAIN OP HUMAN F<ETUS OF 22'5 CM. (BEGINNING OF FIFTH MONTH), SHOWING THE 

 PARTS OF THE DEVELOPING RHINENCEPHALON APPARENT ON THE BASAL SURFACE. (After 

 Retzius.) 



OLFACTORY Kl'LU 



LATERAL OLFACTORY 

 GYRUS (STRIA) 



POSTERIOR PAROLFACTORY 

 SULCUS 



UNCUS (HIPPOCAMPAL 

 GYRVS) 



MEDIAL OLFACTORY 

 i (STKJA) 



OLFACTORY TRACT 



7.7.V/;.V 7.V.ST/..7; 



ANTERIOR PERFdl! I //,/' 

 SUBSTANCE 



HIPPOCAMPAL GYRUS 



Morphologically, the rhinencephalon may be divided into an anterior and a pos- 

 terior portion. 



The olfactory lobe proper may be considered as comprising the anterior portion of 

 the rhinencephalon. This belongs almost wholly to the base of the encephalon, and 

 consists of the following parts: 



(1) The olfactory bulb is an elongated, oval enlargement of grey substance which 

 lies upon the lamina cribrosa of the ethmoid bone, and, practically free, it presses 

 under the anterior end of the olfactory suleus of the basal surface of the frontal lobe. 

 The numerous thin filaments of the olfactory nerve enter the cranium through the 

 foramina of the lamina cribrosa and pass mto the ventral surface of the bulb. 



(2) The olfactory tract is a triangular band of white substance which arises in 

 the olfactory bulb, and continues backwards about 20 mm. to the region of the 

 anterior perforated substance. It appears triangular in transverse section from the 

 fact that its upper side fits into the olfactory sulcus. It becomes somewhat broader 

 at its posterior end. 



(3) The olfactory trigone is the small triangular ridge joining in front the 

 anterior perforated substance. Upon it the olfactory tract breaks up into three 

 roots, the lateral, intermediate, and medial olfactory strife. The lateral olfactory stria 

 emphasizes the lateral portion of the trigone into what is sometimes called the lateral 

 olfactory gyrus, a portion of which is directly continuous into the limen insult: 

 (figs. 624 and 627) . While a few of the fibres of the lateral stria penetrate this portion, 



