THE RHINENCEPHALON 277 



in 1890, and possess considerable theoretic and historic interest. Since in these 

 glomeruli the olfactory nerve-fibers come into contact with only the dendritic 

 ramifications of the mitral and tufted cells, it is evident that these dendrites 

 must take up and transmit the olfactory impulses. That is to say, these glomer- 

 uli furnished positive proof that the dendrites are not, as had been thought by 

 many investigators, merely root-like branches which serve for the nutrition of 

 the cell. The mitral cells are larger than the tufted cells and their axons are 

 thicker. These coarse axons are directed for the most part into the lateral ol- 

 factory stria; while the finer axons of the tufted cells pass through the anterior 

 commissure to the opposite olfactory bulb (Fig. 207). The axons of the deeply 

 placed granule cells are relatively short and are directed toward the surface of 

 the bulb. 



The olfactory tract consists of fibers passing to and from the olfactory bulb. 

 Through it each bulb receives fibers from the other by way of the anterior com- 

 missure as well as from the hippocampal cortex. The fibers leaving the olfac- 

 tory bulb are the axons of the mitral and tufted cells. By far the greater number 

 of the axons of the mitral cells are continued into the lateral olfactory stria. A 

 much smaller number terminates in the olfactory trigone and in the tuberculum 

 olfactorium within the anterior perforated substance. Other fibers are said to 

 pass by way of the medial olfactory stria to the parolfactory area of Broca, to 

 the subcallosal gyrus, and to the septum pellucidium, but this is open to ques- 

 tion. The fibers of the lateral olfactory stria run upon the surface of the lateral 

 olfactory gyrus, also known as the frontal olfactory cortex, to which they give 

 off collaterals (Fig. 207). The terminal fibers reach the uncus and part of the 

 hippocampal gyrus. The chief olfactory centers of the second order are, there- 

 fore, found in the pyriform area. 



According to Cajal (1911), the hippocampal gyrus may be subdivided in man, as in the 

 mammals, into five areas: (1) the external region near the rhinal fissure; (2) the principal 

 olfactory region, the most salient part of the convolution; (3) the presubiculum, a transitional 

 area between 2 and 4; (4) the subiculum, near the hippocampal fissure, and (5) the caudal 

 olfactory region, including the caudal part of the hippocampal gyrus. Of these five regions, 

 Cajal finds fibers from the lateral olfactory stria going to the second or principal olfactory 

 region only. The presubiculum and subiculum and the caudal olfactory region represent 

 olfactory association centers. The subiculum is characterized by the presence of a thick 

 layer of myelinated fibers upon its surface. 



The hippocampus, which constitutes an olfactory center of a still higher 

 order, is directly continuous with the portion of the hippocampal gyrus known 

 as the subiculum (Fig. 209), and is formed by a primitive portion of the cortex 



