CORPORA QUADRIGEMINA. 473 



THE CORPORA QUADRIGEMINA. 



The corpora quadrigemina are four small grayish eminences 

 situated beneath the posterior border of the corpus callosum and be- 

 hind the third ventricle. They rest upon the lamina quadrigemina, 

 which forms the roof of the aqueduct of Sylvius. The anterior pair 

 are termed the nates, or the pregemina, the posterior pair the testes, 

 or the postgemina. 



From the external surface of each body there pass outward 

 bundles of fibers termed brachia. The fibers which compose the 

 brachium of the pregeminum pass outward and enter a small col- 

 lection of gray matter, the corpus geniculatum externum, and the optic 

 tract. The fibers which compose the brachium of the postgeminum 

 are divided into two bundles, one of which enters a second small 

 collection of gray matter, the corpus geniculatum internum, while the 

 other passes forward beneath this body to enter the internal capsule, 

 beyond which it passes to the cortex of the temporal region of the 

 cerebrum (Fig. 212). 



Though these bodies are closely associated anatomically, they 

 differ in origin, in their relations and in their functions. 



Microscopic examination of sections of the quadrigeminal bodies 

 shows 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 adjoining structures. Some of the 

 cells of the pregeminum give off axons which course outward and 

 forward, enter the internal capsule, and pass through the optic 

 radiation to the cortex of the occipital region of the cerebrum. Many 

 fibers of the optic tract, axons of the cells of the retina, end in 

 brush-like expansions around these same cells. There is thus formed 

 a connected pathway between the retina and the occipital cortex. 



The cells of the occipital cortex, however, send axon fibers in the 

 reverse direction through the optic radiation to terminate around the 

 cells of the pregeminum, while axons of pregeminal cells pass for- 

 ward to the retina and to the cells of origin of the third nerve. 



The cells of the postgeminum give origin to axons which pass 

 upward, forward, and outward, enter the internal capsule, and pass 

 by way of the auditory tract to the cortex of the temporo-sphenoidal 

 region of the cerebrum. Many of the fibers of the lateral fillet, a 

 portion of the auditory tract, terminate in brush-like expansions 

 around these same cells. There is thus established a connected 

 pathway between the cochlea and the temporo-sphenoidal cortex. 

 The cells of the temporal cortex, however, send axons in the re- 

 verse direction by way of the auditory tract to the cells of the 

 postgeminum. There is thus established a double communication 

 between the occipital and temporal region of the cerebral cortex, 

 and the pregeminal and postgeminal bodies respectively. 



