THE CENTRAL NERVOUS SYSTEM. 323 



respectively with the anterior and the posterior corpora quadrigemina 

 by means of the corresponding brachia. 



The lateral or external geniculate bodies exhibit a character- 

 istic structure, consisting of alternate layers of white and gray matter ; 

 the white striae are composed largely of fibres derived from the 

 optic tracts, the gray zones probably also receiving collateral con- 

 nections from the retina. The nerve -cells of the gray matter, many 

 of which are large and deeply pigmented, send axis-cylinder processes 

 as far as, possibly, the occipital cortex. The mesial or inner 

 geniculate bodies contain numerous nerve-cells of medium size, 

 together with fibres seemingly connected with the mesial root of the 

 optic tract ; an intimate relation between these bodies and the optic 

 fibres, however, is questionable. 



The masses of gray matter forming the structures along the fore 

 part and the floor of the third ventricle, including the lamina 

 cinerea, tuber cinereum, infundibulum, and posterior per- 

 forated space, contain scattered ganglion-cells, together with 

 certain special bundles of nerve-fibres. The corpora mam- 

 millaria are composed of bundles of fibres, and contain gray nuclei 

 within the superficial layer of white matter. The hollow coni- 

 cal infundibulum bears at its lower extremity the pituitary body, 

 with part of which the funnel-shaped extension of the cerebral vesicle 

 is at one time continuous. 



THE OLFACTORY LOBE. 



The structures described in human anatomy as the olfactory 

 nerves represent the rudimentary olfactory lobes, which in many 

 of the lower animals constitute conspicuous divisions of the brain. 

 This lobe, as found in man, comprises three parts, the tuber olfac- 

 torium, the tractus olfactorius, and the bulbus olfactorius. 



The tuber olfactorium does not here call for attention, since its 

 adequate consideration lies without the purpose of these pages. 



The tractus olfactorius, on transverse section, exhibits zones 

 of white and gray matter, together with a central flattened area 

 of neuroglia, indicating the position of the obliterated lumen, 

 which in the embryonic condition temporarily, and in the lower 

 animals permanently, existed as a continuation of the ventricular 

 cavity. 



The gray substance is richest in the dorsal part of the tract, 

 where it forms an oval area surrounded by medullated nerve-fibres ; 

 these latter become continuous at the lateral angles with the thick 

 medullary fibre-layer occupying the ventral zone, the juncture 

 between the two being marked by a thickening of the medullary 



