THE DIENCEPHALON. 1129 



each body is seen to be composed of an outer layer of white matter enclosing a core 

 of gray substance, known collectively as the nucleus mammillaris. The latter is 

 subdivided into a medial and lateral part by fibres from the downward arching ante- 

 rior pillar of the fornix, which penetrate the gray matter as well as invest to a large- 

 extent its exterior. Only a part of (i) the > fornix fibres, however, end directly in 

 the mammillary nuclei, since some pass above and behind the ganglion to gain the 

 hypothalamic commissure (page 1128) and, after decussation, to end in the mam- 

 millary body of the opposite side. From the dorsal part of the medial nucleus, 

 distinguished from the lateral one by its larger nerve-cells, emerges a distinct and 

 compact bundle of fibers (Fig. 967), which on clearing the nucleus, separates into two 

 strands. One of these, known as (2) the mammillo-thalamic tract, or the bundle 

 of Vicq d 1 Azyr, courses upward and forward, and ends within the anterior nucleus of 

 the thalamus ; in this manner it completes the paths by which the cortical olfactory 

 centres within the hippocampus major are connected (by way of the fimbria, body 

 and anterior pillar of the fornix and the mammillo-thalamic strand) with the thalamus 

 (Fig. 1049). That fibres pass between the latter and the mammillary nucleus in 

 both directions, is shown by the fact that destruction of either of these centres is fol- 

 lowed in turn by ascending or descending degeneration of the fibres. (3) The 

 other part of the bundle issuing from the mammillary nucleus arches backward and 

 downward and, as the mammillo-tegmental tract, is traceable into the tegmentum of 

 the mid-brain to the vicinity of the inferior colliculus. (4) Under the name, pedun- 

 culus corporis mammillaris, another mammillo-tegmental tract is described. This 

 strand springs from the lateral mammillary nucleus, and, coursing backward and 

 downward along the medial margin of the crusta, enters the tegmentum. Its des- 

 tination is uncertain, but according to Kolliker the tract probably ends in the central 

 gray matter surrounding the Sylvian aqueduct in proximity with the trochlear 

 nucleus. Other, but much less well established, strands have been described by 

 Lenhossek as proceeding forward from the peripheral layer of the mammillary body 

 over the tuber cinereum. Concerning their further course little is known with 

 certainty. 



The tuber cinereum is the first of a series of median outpouchings which 

 model the thin sheet of gray matter constituting the floor and the anterior wall of 

 the third ventricle and belong to the pars optica of the hypothalamus. As seen from 

 the exterior (Fig. 993), the tuber cinereum is a median elevation placed between the 

 mammillary bodies behind and the optic chiasm in front, and the cerebral peduncles 

 and the optic tracts at the sides. Together with the infundibulum, it forms the most 

 dependent part of the third ventricle and consists of a thin layer of gray matter, less 

 than 1.5 mm. thick, that is continued forward as the attenuated extension of the im- 

 portant sheet found within the mid-brain and fourth ventricle. In addition to the fibre- 

 strands coming from the mammillary bodies noted by Lenhossek, this investigator 

 and Kolliker credit the tuber cinereum with possessing small paired composite gang- 

 lia, the nuclei tuberis and the nuclei supraoptici of Kolliker. Concerning their con- 

 nections nothing is definitely known. The anterior part of the tuber, immediately 

 behind the optic chiasm, descends abruptly and somewhat forward to form a funnel- 

 shaped stalk, the infundibulum, to whose lower end or apex is attached the pos- 

 terior lobe of the pituitary body (Fig. 976). Although in the very young child the 

 infundibulum retains to some extent its original character as a hollow outgrowth 

 from the ventricle, in the mature subject this cavity, the recessus infundibuli, 

 has mostly disappeared and the stalk is solid, save for a slight diverticulum within its 

 upper and widest part. 



The posterior part of the tuber cinereum, between the root of the infundibulum 

 and the mammillary bodies, exhibits occasionally in the adult brain, and almost con- 

 stantly in that of the foetus, a small rounded median projection, flanked on each side 

 by a slight elevation. To this modelling Retzius has applied the name, eminentia 

 saccularis in recognition of its similarity to the evagination (saccus vasculosus) found 

 in fishes. The eminence encloses a shallow pouch, recessus saccularis, which opens 

 into the third ventricle. 



The pituitary body (hypophysis cerebri) is attached to the dependent tip of the 

 infundibulum, and, closely invested by a loose sheath of connective tissue, hangs 



