THE RHINENCEPHALON 



865 



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. 



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

 posterior division. 



The anterior division. — the olfactory lobe proper, 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 sulcus in the basal surface of the 

 frontal lobe. The numerous thin filaments of nonmedullated axones of the 

 olfactory nerve enter the cranium through the foramina of the lamina cribrosa and 

 pass into the ventral surface of the bulb. 



Fig. 680. — Brain of Human Fcetus of 22.5 Cm. (Beginning of Fifth Month), showing 

 THE Parts of the Developing Rhinencephalon Apparent on the Basal Surface. 

 (After Retzius.) 



Olfactory bulb 



Lateral olfactory gyrus (stria) 



Posterior parolfactory siilcus 



Uncus (hippocampal gyrus) 



Medial olfactory gyrus (stria) 



Olfactory tract 



Limen insulse 



Anterior perforated substance 



Hippocampal gyrus 



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

 the olfactory bulb, and continues backward 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 (olfactory tubercle) is the small triangular ridge, the 

 posterior continuation of the olfactory tract joining the anterior perforated sub- 

 stance. In it the olfactory tract breaks up into three roots, the lateral, in- 

 termediate, and medial olfactory strice (gyri). The lateral olfactory stria em- 

 phasizes the lateral portion of the trigone into the lateral olfactory gyrus, a portion 

 of which is directly continuous into the limen insula; (figs. 676, 680). 



While a few of the fibres of the lateral stria penetrate this region, the greater mass of them 

 pass obhquely lateralward over it and gradually disappear in the antero-lateral portion of the 

 anterior perforated substance, in which some of them terminate, but through which most of 

 them pass to curve into the anterior end of the hippocampal gyrus and terminate there, chiefly 

 in the uncus. In most of the mammals the lateral stria is so strong that it appears as a super- 

 ficial white band passing directly into the uncus. In the early foetus it is seen to enter the 

 uncus in two branches, forming the medial semilunar gyrus and the lateral gyrus ambiens upon 

 the imcus. A portion of the li7nen insulce belongs to the rhinencephalon. 



(4) The parolfactory area (Broca's area) involves the mesial extension of the 

 olfactory trigone, and is concerned with the medial olfactory stria. On the basal 

 surface of the hemisphere this area involves the posterior extremity of the gyrus 

 rectus — a portion of which is sometimes separated from the remainder of the gyrus 

 by a ventral prolongation of the anterior parolfactory sulcus of the medial surface 

 (see figs. 679, 706). This prolongation when present has been called the 

 fissura serotina. On the medial surface the parolfactory area appears as a 



