166 



ANATOMY OF THE CENTRAL NEHTOUS SYSTEM. 



of the septum, divides the whole inner side into a dorsal and a ventral por- 

 tion. Only the dorsal portion is covered with cortical tissue. This sulcus, 

 which thus forms the ventral boundary of the brain-cortex, persists through- 

 out the whole animal series after its appearance in the reptiles. In the 

 accompanying figure (114) it is called Fissura arcuata septi, but in mammals 

 it is called the "inner marginal fissure." 



Into the cortex which covers the dorsal portion of the septum an im- 

 portant bundle regularly enters: the Tr. olfadorius septi. It arises from 

 the olfactory apparatus of the base of the brain; its fibers converge toward 

 the median brain-surface, thence pass upward and backward into the cortex. 

 The region in which it ends is, on this account, called the olfactory cortex. 

 In amphibia, with uncertainty, demonstrable; this bundle is always promi- 



^plcUum 



Fig. 115. — Sagittal section through the brain of a^ cliicken. 



nent in reptilia and mammalia (see Fig. 114). In birds, however, it is veiled 

 by a bundle, which is especially developed in these animals — Tr. septo- 

 mesencephalicus — and which, arising in a wide origin from the dorsal portion 

 of the cortex near its edge where it turns outward, is spread out upon the 

 inner surface of the avian brain like a broad, white fan. Eeaching the base 

 of the brain, it encircles it with externally directed fibers, and just anterior 

 to the optic tract, which it reaches on the lateral aspect of the brain, passes 

 again upward and backward to disappear in the most anterior portion of the 

 roof of the midbrain. Thus this tract connects the midbrain with a par- 

 ticular cortical region (Fig. 115). It is foreshadowed in reptiles, but has 

 not been located in mammals. 



If one makes a frontal section through the forebrain of any of the 



