THE CORPORA QUADRIGEMINA 569 



The aqueduct of Sylvius is a short narrow canal which connects the cavity 

 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 colliculi 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. 



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. 



External and somewhat inferior to the corpora quadrigernina 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 external and the 

 internal geniculate bodies, with the corresponding quadrigeminal bodies 

 are known as the superior and inferior brachia respectively. 



The External Geniculate Body. 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 Internal Geniculate Body. 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 decussation is in the posterior part of 

 the optic chiasm. The internal geniculate body receives fibers from the 

 auditory tract. 



