NERVOUS SYSTEM 845 



surfaces so that they are exposed to view only by dissection. A part of 

 the original hippocampus is then vestigial. 



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

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

 (where the hippocampus ends) with the hypothalmus. In the mam- 

 mals, by the flexure of the cerebrum, this same band of fibers, here called 

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

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

 forward, below the corpus callosum, and then down, in front of the in- 

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

 on the floor of the hypothalmic region." 



Headward, on the dorsal side, the walls become somewhat thick- 

 ened, bulging out into a pair of prominences known as the optic lobes 

 or corpora bigemina in the lower forms of animals, while in the mam- 

 mals there are two such pairs of lobes which are therefore called corpora 

 quadrigemina (Fig. 473). The roof of this region remains comparatively 

 thin, but the floor becomes somewhat thicker and forms the cerebral 

 peduncle. Connecting the mid-brain with the hind-brain is a short con- 

 stricted area known as the isthmus. From here running caudad along 

 each lateral wall there is often a groove (seldom, if ever, seen in the 

 adult) called the limiting sulcus or the sulcus of Monro. This naturally 

 divides the brain and spinal cord from here to the tail-end into a dorsal 

 and a ventral half, a fact that is of considerable importance, because the 

 entire dorsal area is sensory while the ventral is motor in character. 

 Further, in the study of the central nervous system's development it is 

 the dorsal portion in which most of the changes come, comparatively 

 few developing on the ventral side. 



The hind-brain is again divided. The part lying cephalad develops 

 Into the cerebellum or balancing brain (organ of coordination), while 

 the caudal end tapers rather gradually and is known as the myelencepha- 

 Ion or medulla oblongata. The cavity in the hind-brain, most of which 

 is located within the medulla, is known as the fourth ventricle, while the 

 small lumen which connects the third and fourth ventricle is called the 

 aqueductus cerebri or the aqueduct of Sylvius (Fig. 282). 



It will, therefore, be noticed that from the earlier three compart- 

 ments of the head end of the brain and spinal cord there have developed 

 five brain divisions with four ventricles. All the ventricles form a con- 

 tinuous open space throughout the entire central nervous system. 



The roof plate in the region of the cerebellum, which originally was 

 quite thick, forces the most cephalic portion of the two dorsal zones 

 far apart, so that they then become quite thin and broad, whereas the 

 floor plate becomes greatly thickened and constitutes the pyramids which 

 pass in front into the cerebral peduncles. 



A comprehensive study of the brain is a tedious and difficult task 

 and requires a very thorough going over, and remembering the main 



