744 THE NERVOUS SYSTEM. 



which is made up of fine nerve-fibres, coursing in a longitudinal direction, and 

 containing between them small masses of gray substance, consisting of small mul- 

 tipolar nerve-cells imbedded in gray matter. Lastly, between this body and the 

 central gray matter around the Sylvian aqueduct is a thin lamina of white matter, 

 derived from the upper fillet. 



The geniculate bodies are continuous with the gray substance of the optic 

 thalamus, and the external one (corpus genjculatum extcrnum] is peculiar on 

 account of its dark color, due to its cells containing pigment. It presents 

 a laminated arrangement, and consists of alternate thick layers of gray 

 matter and tkin layers of white matter. Its cells are multipolar. The internal 

 body (corpus geniculatum internuni) is of lighter color, does not present a lami- 

 nated arrangement, arid its cells are smaller in size and fusiform in shape. 

 These bodies, strictly speaking, belong, the external to the inter-brain, and the 

 internal to the mid-brain. 



The locus niger, or gray matter of the crus cerebri, like the external geniculate 

 body, is peculiar from the large number of dark pigment-granules which are 

 contained in its ganglion-cells, and which give to it its dark color, from which it 

 has derived its name. Its cells are small and multipolar. 



The Aqueduct of Sylvius. This is the " ventricle " of the mid-brain. It is a 

 narrow tube into which the fourth ventricle opens below, and which opens into 

 the third ventricle above. Hence it is sometimes called the iter a tertio ad 

 quartum ventriculum. It is a little over half an inch long. It lies in the teg- 

 mentum, and its course is upward and forward, the same direction as that of the 

 dorsal surface of the tegmentum, on which the groove between the right and left 

 corpora quadrigemina indicates its position. It lies immediately ventral to this 

 groove. Its roof is the lamina quadrigemina, the white layer which supports the 

 corpora quadrigemina, and into which are prolonged the fibres of the superior 

 medullary velum. Its shape, on transverse section, varies, being T-shaped near 

 the fourth ventricle, shield-shaped about midway in its course, and triangular 

 near the third ventricle, into which it opens just at the bend between end of 

 dorsal surface and beginning of the superior surface of the mid-brain. 



In all cross-sections through the aqueduct /. e. at right angles to the plane 

 of the dorsal surface, from its beginning to end a large amount of tegment'tl 

 tissue is to be seen between it and the ntfatantia niqra, ventrally. Hence the 

 latter can have nothing to do with the formation of the floor of the aqueduct. 



The central gray matter surrounding the Sylvian aqueduct presents some 

 features requiring especial mention. It forms a tolerably thick layer surround- 

 ing the canal, but is thicker on the lower wall that is, below the canal than 

 above. The cells, which are multipolar, are here collected into groups, and form 

 nuclei for the origin of the third and fourth cranial nerves. The nucleus for the 

 third and fourth consists of a, column of cells of large size on either side of, and 

 close to, the median line. In addition to these cells there are found at the per- 

 iphery of the zone of gray matter surrounding the aqueduct some other, and 

 larger, cells, sometimes single, sometimes grouped in tAvos or threes, or even 

 more. They are globular, and lie in the midst of well-marked nerve-fibres, with 

 which their processes appear to be continuous. Close to the Lateral margin of 

 this gray matter, in its lower part, is the upper end of the upper nucleus of the 

 fifth nerve. 



The third nerve passes from its nucleus in a somewhat curved manner through 

 the tegmentum, and emerges from the oculo-motor groove on the inner margin of 

 the crusta. Some of its fibres, however, from the dorsal part of the nucleus, 

 decussate. 



The fourth nerve passes downward from its nucleus toward the pons, on enter- 

 ing which it turns dorsally, and then runs transversely in the superior medullary 

 velum, crossing the middle line and decussating with its fellow, to emerge from 

 the dorsal surface of the velum. It then curves obliquely downward and for- 

 ward, resting on the crusta. 



