85b COMPARATIVE ANATOMY 



rior commissure of the twixt-brain. All of these structures in the higher 

 vertebrates are completely covered by the cerebral hemispheres growing 

 backward over them. In many of the extinct reptiles there are large 

 parietal foramina and it is supposed that these animals therefore had 

 w r ell developed parietal or pineal organs. Directly behind the lamina 

 terminalis there is a chorioid plexus located in the fourth ventricle. This 

 comes from the roof of the brain in this region and a part of it invades 

 the third ventricle, while another part, the inferior plexus, sends branches 

 through the interventricular foramina into the lateral ventricle. This 

 provides a blood supply to the interior portions of the cerebral hemi- 

 spheres.' 



A funnel-shaped protrusion from the floor of the diencephalon may 

 be seen posterior and ventral to the optic chiasma known as the infundib- 

 ulum. This connects with the pituitary body or hypophysis, which latter 

 organ has developed from the mouth region. It is encased by the devel- 

 oping skull in a little bony case of its own called the sella turcica 

 (Turkish saddle). The epithelium of the mouth from which the hypo- 

 physis springs, remains connected for a time to that organ and the point 

 of ingrowth into the brain is called Rathke's pocket (Fig. 301, 1). It 

 will be noticed that the pituitary body grows upward from the oral 

 cavity just mentioned, while the infundibulum grows downward from 

 directly behind the optic chiasma to meet it. There are really two parts 

 to the pituitary body, both rich in blood and lymph vessels. The organ 

 is known as a gland of internal secretion. Its action is supposed to be 

 connected with the fat-storing powers of the animals ; sometimes there 

 is to be found a postoptic commissure connecting the ventral parts of the 

 brain in this region. 



MESENCEPHALON 



The mesencephalon or mid-brain, as has already been stated, does 

 not change very much from the way it appears in the embryo. On the 

 dorsal surface there are two lateral swellings, the optic lobes. In the 

 mammals these are transversely divided and are called the corpora quad- 

 rigemina. If they do not divide transversely they are called corpora 

 bigemina. Each optic lobe is connected with fibers from the eye on the 

 opposite side to which it itself is located. In the fishes the ventricle of 

 the mid-brain is quite large and extends into the optic lobes, but in the 

 higher groups of vertebrates the ventricle is reduced to a very small 

 opening or aqueduct. At the anterior end of the dorsal body of the mid- 

 brain a band of nerve fibers crosses from one side to the other. Any 

 such cross connections are called commissures. These connect the two 

 sides of the central nervous system. Cross fibers of this kind are very 

 numerous in the spinal cord and there are also several in the brain. This 

 particular one we have just mentioned is called the posterior commissure. 



