mo HUMAN ANATOMY. 



lateral and median grooves of the mid-brain. Concerning the functions and connec- 

 tions of^the neurones within the substantia nigra very little is known. 



The Quadrigeminal and Geniculate Bodies. — The inferior colliculus 

 consists chiefly of a biconvex (in section oval) mass of gray matter, the nucleus 

 colliculi inferioris, in which many nerve-cells of varying form and mostly of small 

 size lie embedded within a complex of nerve-fibres. The lower end of the nucleus 

 stands in intimate relation with the acoustic fibres composing the lateral fillet, many 

 of which enter the ventral aspect of the nucleus colliculi to end around its cells, whilst 

 a considerable number pass superficial to the nucleus and thus form an external fibre- 

 layer that intervenes between the gray nucleus and the surface. Although many of 

 these external fillet-fibres enter the colliculus at higher levels, not a few continue, 

 by way of the inferior brachium, to the median geniculate body, around whose neu- 

 rones they end. A much smaller and less well defined tract of fillet-fibres passes to 

 the mesial side of the nucleus, the ventral margin of which is thus embraced (Fig. 960) 

 by the diverging but unequally robust fillet-strands that in this manner partially 

 encapsulate the coUicular nucleus. From the supero-lateral parts of the nucleus 

 fibres proceed which, in conjunction with those continued from the lateral fillet, 

 form the chief constituents of the inferior brachium. A part of this arm, how- 

 ever, is composed of strands of fibres that pass from the cerebral cortex (especially 

 the temporal) to the inferior coUiculus. Towards the upper' pole of the nucleus 

 some loose strands of fillet-fibres, probably along with commissural fibres uniting 

 the inferior colliculi, cross the mid-line and establish a decussation. 



The internal or median geniculate body (corpus geniculatum mediale), 

 although genetically belonging to the diencephalon, is so closely related to the 

 inferior colliculus as to require description in this place. It consists of a superficial 

 layer of white matter composed of fibres from the inferior brachium, which pass 

 outward as continuations of the lateral fillet, as axones of the cells of the inferior 

 colliculus, or as fibres forming the mesial root of the optic tract, also known as the 

 inferior commissure of Gudden. Within this fibre-capsule lies an oval mass of 

 gray matter, the nucleus corporis geniculati medialis, from whose cells axones 

 proceed chiefly towards the cerebral cortex in continuation of the auditory paths 

 of which the inferior colliculus and the median geniculate body are important 

 stations. 



Connections of the Inferior Colliculus and Median Geniculate Body. — Mention has been 

 made, when describing the reception-nuclei of the cochlear portion of the auditory nerve (page 

 1076) , that the tract of the lateral fillet takes origin to an important extent from the cells of these 

 nuclei, and, further, (page 1082), that the fillet-fibres end around either the cells of the inferior 

 colliculus, or those of the median geniculate body. It is evident, therefore, that these parts of 

 the mid-brain stand in intimate relation with the parts concerned in conveying auditory impulses. 

 The more detailed account of the chaining together of the neurones forming such paths is 

 deferred until the auditory nerve is considered (page 1257). The connection of the fibres com- 

 posing the median root of the optic tract with the median geniculate body and the inferior collic- 

 ulus has been established beyond doubt ; further, that this part of the optic tract is not concerned 

 in conducting visual impulses, is shown by the fact that these fibres remain unaffected under 

 conditions (after removal of the eyes) that lead to degeneration of the fibres of retinal origin. 

 The destination and significance of the fibre-systems included within the median root of the 

 optic tract are only imperfectly understood, but it may be accepted as certain that they can no 

 longer be regarded as merely establishing a bond between the median geniculate and indirectly 

 the inferior quadrigeminal bodies of the two sides, as implied by the name commissure, since 

 many of these fibres are probably directed after decussation to the lenticular nucleus (globus 

 pallidus), while others possibly may end on the same side in the subthalamic nucleus (page 1128). 

 The gray matter of the inferior colliculus, like that of the superior, gives rise to fibres of the 

 tecto-bulbar and tecto-spinai tracts, presently to be described (page iiii). 



The superior colliculus is composed of a number of alternating layers of white 

 and gray matter. The latter, however, is not aggregated into a definite nucleus, as 

 in the case of the inferior colliculus, but is broken up into uncertain zones by the 

 tracts of nerve-fibres. Although as many as seven layers have been described, 

 some of these are so blended that only four well-defined strata can be readily 

 distinguished. From the surface inward these are : 



