556 TEXT-BOOK OF PHYSIOLOGY 



of the fourth with the cavity of the third ventricle. It is lined by the epen-' 

 dyma and surrounded by a layer of gray matter continuous with that forming 

 the floor of the fourth ventricle. In that portion of the gray matter lying 

 beneath or ventral to the aqueduct there are groups of nerve-cells which 

 give origin to axons which unite to form the third and fourth cranial nerves. 



THE CORPORA QUADRIGEMINA 



The corpora quadrigemina are four small grayish eminences situated 

 beneath the posterior border of the corpus callosum and behind the third 

 ventricle. They rest upon the lamina quadrigemina, which forms the 

 roof of the aqueduct of Sylvius. The superior pair are the larger and are 

 known as the superior quadrigeminal bodies, the superior colliouli or the pre- 

 gemina; the inferior pair are the smaller and are known as the inferior quad- 

 rigeminal bodies, the inferior colliculi, or the post-gemina. 



External and somewhat inferior to the corpora quadrigemina are two 

 small collections of gray matter the more external of which has been termed 

 the external geniculate body or the pregeniculum, the more internal of which 

 has been termed the internal geniculate body or the post-geniculum. 



Though these bodies are closely associated anatomically, they differ in 

 origin, in their relations, and in their functions. 



On either side the fibers composing the optic tract pass to and through 

 the geniculate bodies in which some of the fibers- terminate, while others 

 pass onward to the superior and inferior quadrigeminal bodies and there 

 terminate. The bands of white matter associating the superior or external 

 and the inferior or internal geniculate bodies, with the corresponding quad- 

 rigeminal bodies are known as the superior and inferior brachia respectively. 

 The internal geniculate body gives origin to and receives fibers from the 

 mesial portion of the optic tract which is in reality not a portion of the optic 

 tract proper, but a commisural band (Gudden) which associates the body 

 from which it arises with that of the opposite side. The point of decussa- 

 tion is in the posterior part of the optic chiasm. 



The external geniculate body is a terminal station for a portion of the 

 fine visual fibers coming from the retina. From the cells of this body 

 new axons arise which course forward and upward, enter the internal 

 capsule and pass by way of the optic radiation to the cortex of the occipital 

 region of the cerebrum. 



The corpora quadrigemina show on microscopic examination that they 

 are composed of nerve-cells and nerve-fibers, both of which are so intricately 

 arranged that it is difficult to trace their relation one to another and to ad- 

 joining structures. Some of the cells of the superior quadrigeminal body give 

 origin to axons which pass downward and forward and terminate in brush- 

 like expansions around the nuclei of origin of the oculo-motor, trochlear, and 

 abducent nuclei ; other cells are surrounded by the terminal branches of some 

 of the fibers of the optic tract, though it is not probable that they are true visual 

 fibers. Still other cells receive the terminal branches of axons. the cells of 

 origin of which are located in the occipital cortex of the cerebrum and which 

 reach the superior quadrigeminal body by way of the optic radiation and 

 internal capsule. 



The cells of the post-geminum give origin to axons which pass upward, 



