THE RHINENCEPHALON 



267 



The medial olfactory gyrus and stria require further investigation. It has been gen- 

 erally supposed that the stria is formed by olfactory fibers of the second and third order 

 running to the olfactory centers in the rostral part of the medial surface of the hemisphere. 

 These are certainly few in number in the higher mammals, and Cajal (1911), who worked 

 chiefly with rodents, has been unable to identify any such fibers in these animals. The sig- 

 nificance of the medial olfactory gyrus is also obscure. According to Elliot Smith (1915), 

 "the rudiment of the hippocampal formation that develops on the medial surface begins 

 in front alongside the place where the stalk of the olfactory peduncle (which becomes the 

 trigonum olfactorium) is inserted; it passes upward to the superior end of the lamina termi- 

 nalis, from the rest of which it is separated by a triangular mass of gray matter called the 

 corpus paraterminale" (Fig. 200). This description, as well as the figure which accompanies 

 it, suggests a close relation between the rostral end of the hippocampal rudiment and what 

 is ordinarily known as the medial olfactory gyrus. The subdivision of the olfactory lobe 

 into anterior and posterior portions by the morphologically unimportant sulcus parol- 

 factorius posterior, although adopted in the B. N. A., is without justification and leads only 

 to confusion (Elliot Smith, 1907). 



Olfactory bulb 



Lateral olfactory gyrus (stria) 



Posterior parolfactory sulcus 



Amygdaloid nucleus 



Medial olfactory gyrus (stria) 

 Olfactory tract 



Limen insula 



A nterior perforated substance 



Hippocampal gyrus 



Fig. 198. Brain of a human fetus of 22.5 cm. Ventral view. (Retzius, Jackson-Morris.) 



Between the olfactory trigone and the medial olfactory gyrus, on the one 

 hand, and the optic tract on the other, is a depressed area of gray matter known 

 as the anterior perforated substance, through the openings in which numerous 

 small arteries reach the basal ganglia (Figs. 172, 197). The part immediately 

 rostral to the optic tract forms a band of lighter color, known as the diagonal 

 gyrus of the rhinencephalon or the diagonal band of Broca (Fig. 197). This 

 can be followed on to the medial surface of the hemisphere, where it is continued 

 as the paraterminal body or subcallosal gyrus (Fig. 200). Rostral to this gyrus 

 the hippocampal rudiment, which corresponds in part to the parolfactory area 

 of Broca, extends as a narrow band from the rostrum of the corpus callosum 

 toward the medial olfactory gyrus. In those mammals which possess an espe- 

 cially rich innervation of the nose and mouth, the region of the anterior per- 

 forated space is marked by a swelling, sometimes of considerable size, called 



