THE MID-BRAIN. 675 



the olivary nucleus, which pass in a longitudinal direction through the reticular 

 formation of the tegmentum and are continued onward into the internal capsule. 



The red nucleus or nucleus of the tegmentum, is a tract of gray matter situated 

 on either side of the middle line, and is composed of numerous large cells, which 

 are deeply pigmented. It is pierced by the fibres of the third nerve, and pro- 

 longed above into the posterior part of the subthalamic region. 



The Substantia Nigra. This, as already stated, is a layer of deeply pigmented 

 gray matter, which separates the crusta from the tegmentum. It is thicker inter- 

 nally than externally, where it is partially divided up by the mesial fillet passing 

 from the tegmentum to the crusta. It is traversed at its inner part by some 

 of the fibres of origin of the third cranial nerve. The cells are small and mul- 

 tipolar, and are characterized by containing a large amount of dark pigment 

 granules. 



The corpora or tubercula quadrigemina are four rounded eminences placed in 

 pairs, two in front and two behind, and separated from one another by a crucial 

 depression. They are situated on the dorsal surface of the mid-brain, immediately 

 behind the third ventricle and posterior commissure, and beneath the splenium of 

 the corpus callosum. The anterior or upper pair, sometimes called the nates, are 

 the larger. They are oval, their long diameter being directed forward and out- 

 ward, and are of a gray color. The posterior or lower pair, called the tester, are 

 hemispherical in form, and lighter in color than the preceding. From the outer 

 side of each of these eminences, a prominent white band, termed brachium, is con- 

 tinued forward and outward. Those from the nates (brachia anterior a) pass 

 obliquely outward between the pulvinar and the inner geniculate bodies into the 

 external geniculate bodies. Those from the testes (brachia posteriora) lose them- 

 selves beneath an oval prominence on either side of the corpora quadrigemina, 

 called the internal geniculate body. The corpora quadrigemina are larger in the 

 lower animals than in man. In fishes, reptiles, and birds they are hollow, and 

 only two in number (corpora bigemina) ; they represent the anterior quadrigeminals 

 of mammals. In these lower animals the corpora bigemina are frequently termed 

 the optic lobes, because of their connection with the optic tracts. In the mammalia 

 they are four in number, and solid. In the human foetus all four bodies are 

 differentiated by the fifth month, and form at this time a considerable proportion 

 of the brain. 



The corpora quadrigemina are composed of white matter externally, and gray 

 matter within. The posterior pair consist almost entirely of gray matter, covered 

 over by a very thin stratum of white substance. Beneath the gray matter is a 

 thin layer of white fibres, forming a part of the lower fillet. This separates 

 the gray matter of the posterior corpora quadrigemina from the central gray 

 matter of the aqueduct. The anterior pair are covered superficially by a thin 

 stratum of white matter, the stratum zonale, the fibres of which are fine and 

 arranged transversely. Beneath this is the stratum cinereum, a layer of gray 

 matter which resembles a cup, semilunar in shape, thicker in the centre, and 

 thinning off toward the margins, and consisting of numerous multipolar cells, for 

 the most part of small size, embedded in a fine network of nerve-fibres. Below 

 this again is the stratum opticum, or upper gray-white layer, characterized by the 

 large amount of fine nerve-fibres which intersect the gray matter. These fibres 

 vary in size in different parts of the layer, but have for the most part a longitudinal 

 direction. The nerve-cells between the fibres are larger, and send their axis- 

 cylinder processes into the next stratum. Finally there is the stratum lemnisci, 

 or deep gray-white layer, which separates the rest of the body from the gray matter 

 around the aqueduct. It consists of fibres partly derived from the upper fillet 

 and partly from the cells of the preceding layer. Interspersed among these fibres 

 are nerve-cells of large size. 



In close relationship with the corpora quadrigemina are the superior peduncles 

 of the cerebellum. They emerge from the upper and mesial part of the hemi- 

 spheres of the cerebellum, and run upward and forward to the corpora quadn- 



