i6o 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



dorsally on either side into a thin translucent membrane, the septtixn 

 pellucidtun (figs. 165, 167), the space between the two septa (formerly 

 called the fifth ventricle, though it has no relation to the other ven- 

 tricles) is the cavity of the septum. 



There are two tracts of nervous matter which run back, on the medial side 

 of either hemisphere, from the olfactory lobe to the hinder end of the cerebrum. 

 One of these, the hippocampus (fig. 166), passes dorsal, the other, the olfactory 

 tract goes ventral to the foramen of Monro. These two and the associated 

 olfactory matter make up practically all of the pallium (the archipalUum it has 

 been called) in the lower vertebrates, for in these the whole cerebrum is accessory 

 to the sense of smell. But in the mammals, and possibly as low as the reptiles, 

 a part has been added to receive impressions from other somatic senses. This 

 is the neopallium which has grown out lateral to the hippocampus (fig. 163, n) 

 and has become especially large in the higher mammals, in man forming by far 



■^» 



Fig. 166. — Medial side of right hemisphere of turtle (Chrysemys) embryo of 17 

 mm., modified from Johnston. The hippocampal area (A) is darker than rest, b, 

 olfactory bulb; ca, anterior commissure; ch, hippocampal (pallial) commissure; h, 

 hippocampus; oc, optic chiasma; p, general pallium; rp, preoptic recess; t, thalamus, 

 to, olfactory tubercle. 



the greater part of the cerebrum. Its great development has forced the olfactory 

 parts to the medial and lower surfaces where they are not exposed at the fixst 

 glance and parts of them can only be found by dissection. Besides, it has 

 made a part of the original hippocampus vestigial (fig. 165). 



Beginning in the amphibia and reappearing in the reptiles is a tract of 

 fibres on either side, which connects the posterior part of the cerebrum 

 (where the hippocampus ends) with the hypothalamus. In the mammals, 

 by the flexure of the cerebrum, this same band of fibres, here called the 

 fornix, is obliged to take a circuitous course. Starting at the hippocampus 

 on the medial side of the temporal lobe, the fornix runs up, then forward, 

 below the corpus callosum, (fig. 167), and then down, in front of the in- 

 terventricular foramen to end in a protuberance, the corpus mammillare, on 

 the floor of the hypothalamic region. 



