26 



NERVOUS SYSTEM OF VERTEBRATES. 



secondary segments of the brain, is one of the most interesting 

 and important and presents many points of morphological sig- 

 nificance and many variations in different classes of vertebrates. 

 When seen from the side (Figs. 2, 7), it appears as a wedge-shaped 

 segment, the edge of the wedge being upward. The short dorsal 

 wall is mostly membranous, but is thickened at one point by 

 the so-called superior commissure which connects the two small 

 knob-like thickenings of the dorsal border of the lateral wall, 

 the nuclei habenulae. These two bodies which are constantly 



FIG. 9. A diagram of one side of the forebrain of Mustelus cants to show what 

 is believed to be the primitive relations of the wall^and ventricle. 

 FIG. 10. The outline of the ventricles in man. 



present and of great significance in the vertebrate brain, are usually 

 of unequal size on the right and left sides. Just behind the supe- 

 rior commissure there arises from the dorsal surface of the dien- 

 cephalon a small sac or tube which in cyclostomes, many fishes 

 and reptiles extends through the cranium to end beneath the 

 skin on the dorsal surface of the head. This sac in the forms 

 mentioned bears some resemblance in structure to an eye and in 

 several cases is probably functional as a light-percipient organ. 



