92 Anatomy of the Nervous System 



in the human brain, only the latter being recognized as a rule. The 

 tuberculum olfactorium has connections with the pyriform lobe and the 

 septum. Anteriorly, it becomes continuous with the anterior olfactory 

 nucleus (p. 96). Laterally and posteriorly, the praepyriform cortex 

 passes into that of the pyriform lobe, which is distinguished by greater 

 thickness and by a larger number of cell layers. The external granular 

 layer in it is particularly conspicuous, being composed of crowded, deeply- 

 staining cells, while the subjacent layers are more diffuse. It not only 

 receives secondary olfactory fibres, like the praepyriform cortex, but also 

 is the terminus of many tertiary fibres from the latter area as well as of 

 axons from other regions. 



Closely related to the pyriform lobe in function, and 

 covered ventrally and laterally by it, is the amygdaloid 

 complex {amygdala, nucleus amygdalae). This is a group of 

 several distinct nuclei which may have various functions and 

 various origins, but at least some of which — and probably all, 

 according to Johnston — belong to the olfactory system in 

 company with the pyriform cortex. The amygdala certainly 

 has important somatic functions also, however (p. 107). 



The nuclei of this complex are six in number according to Johnston, 

 who names them topographically. A rather large mass is the small- 

 celled central nucleus, directly medial to which is a superficial medial 

 nucleus (Pis. XVI 1 1. -XX.). These are derived from the posterior portion 

 of a ridge of gray matter (the bed of the stria terminalis — p. 96) belonging 

 to the primitive olfactory apparatus. A conspicuous, rounded group of 

 rather larger cells superficially situated anterior to the medial nucleus is 

 also a remnant of the latter apparatus. This is the nucleus of the lateral 

 olfactory tract {praesubicular area) (PI. XX.). Ventral to the central 

 nucleus, and pushing partly in between it and the medial nucleus, is the 

 basal nucleus (Pis. XVIII., XIX.), which is less well developed in the rat, 

 where it is composed of small cells, than in some other mammals, where 

 it has distinct large- and small-celled parts. Dorso-lateral to the basal, 

 and central nuclei is a distinct mass lying between the external capsule 

 and the ventricular surface, the lateral nucleus (Pis. XVIII. -XX.). The 

 basal and lateral nuclei are of more recent origin than those previously 

 mentioned and have been derived from the pyriform cortex by infolding 

 or immigration of cells along the line of the endorhinal groove. Finally, 

 there is a cortical nucleus in a superficial position between the medial 



