2 g THE NERVOUS SYSTEM 



axons are directed into the layer of pyramidal cells of the hippocampus. Orig- 

 inally this layer of pyramidal cells was continuous with the granule layer of 

 the fascia dentata, but in all the higher mammals a break in this cellular stratum 

 has occurred at the point of transition between the two divisions of the archi- 



pallium. 



THE OLFACTORY PATHWAYS 



Impulses reach the glomeruli of the olfactory bulb along the fibers of the 

 olfactory nerve and are here transferred to the dendrites of the mitral cells. 

 Axons arising from these cells and running in the lateral olfactory stria transmit 

 the impulses to the pyriform area (Fig. 207), whence they are conveyed to the 

 hippocampus and fascia dentata by fibers entering the molecular layer in both 

 of these parts of the hippocampal formation (Fig. 210). 



According to Cajal, the fibers of the lateral olfactory stria terminate in the principal 

 olfactory region of the hippocampal gyrus, and there are present within the cortex of the 

 pyriform area sagittal association fibers which unite the principal olfactory region with the 

 caudal olfactory region of the hippocampal gyrus. From this latter region fibers reach the 

 hippocampus and fascia dentata. These are relatively thick fibers which are found at first 

 in the angle of the subiculum and can be traced through all the layers of that center into 

 the molecular layer of the hippocampus and fascia dentata (Fig. 210, B). Within the molec- 

 ular layer the impulses are transferred from these fibers to the dendrites of the pyramidal 

 and granule cells. It was formerly supposed that fibers from the trigonum olfactorium, 

 substantia perforata anterior, and septum pellucidum reached the hippocampus through 

 the strize longitudinales and the fornix, and served as the chief conductors of afferent im- 

 pulses toward the hippocampus. But according to Cajal, "The hippocampus does not receive 

 olfactory impulses from the frontal region of the brain, nor through the intermediation of the 

 septum pellucidum." 



The efferent fibers from the hippocampus represent the axons of the pyra- 

 midal cells. These penetrate the stratum oriens and enter the alveus (Fig. 

 210). Thence they are continued into the fimbria and fornix. They include 

 both commissural and projection fibers. The commissural fibers serve to unite 

 the two hippocampi and run through the hippocampal commissure as the trans- 

 verse fibers of the psalterium. The projection fibers are continued rostrally; 

 and in their course through the body of the fornix they form on either side of 

 the median plane a longitudinal bundle, which is continued into the columna 

 fornicis (Fig. 203). The latter bends caudally into the hypothalamic region, 

 giving off fibers to the tuber cinereum and the mammillary body. The remaining 

 fibers of the columna fornicis undergo a decussation just behind the mamillary 

 body and are continued in the reticular formation of the brain stem as far, at 

 least, as the pons. It will be obvious that the fornix is the efferent projection 



