908 THE NERVE SYSTEM 



missure (supracommissure of Osborn). The ventral stalk is folded over another 

 commissural band the posterior commissure. 



Structure. The pineal body consists of a number of follicles, lined by epithelium and 

 connected by ingrowths of connective tissue. The follicles contain a transparent viscid fluid 

 and a quantity of sabulous matter named brain sand (acervus cerebri), composed of phos- 

 phate and carbonate of lime, phosphate of magnesia and ammonia, with a little animal 

 matter. These concretions are almost constant in their existence, and are present at all periods 

 of life. 



In the interval between the pineal body and the caudal end of the thalamus 

 lies a small triangular depression (sometimes an elevation) known as the tri- 

 gonum habenulae, marking the position of the nidus, or ganglion habenulae, a group 

 of small angular cells. The axones from these cells are collected ventrad into the 

 fasciculus retroflexus (Meynert), which courses through the tegmentum mesad of 

 the red nucleus to end in the interpeduncular ganglion (Gudden) in the posterior 

 perforated substance. In addition to this fasciculus, the habenula is the reunion 

 point for two other sets of fibres: (1) the stria medullaris and (2) habenular 

 proper, or habenular commissure. The stria medullaris (p. 905) is made up of 

 axones arising from two sources: (1) cells in the hippocampus (via fornix) and 

 (2) cells in the ganglion opticum basale. These join near the anterior pillar of 

 fornix (fornicolumn) and run caudad on the mesal thalamic surface, to end 

 in the habenular ganglion of the same side and, by crossing in the dorsal stalk 

 of the epiphysis from the habenular commissure, ending in the corresponding 

 nidus habenulae of the opposite side. 



The posterior commissure is a round band of white fibres crossing from side 

 to side in the ventral stalk of the pineal body bridging the aqueduct at its continua- 

 tion into the third ventricle. The posterior commissure shares relation with both 

 fore-brain and mid-brain structures and is formed of decussating fibres which 

 may be enumerated in the following systems: (a) fibres arising in the special 

 nucleus (described on p. 901) for the medial longitudinal bundle; (&) fibres con- 

 necting the two thalami; (c) fibres connecting the habenular nidi; (d) fibres connect- 

 ing the superior quadrigemina. 



[NOTE. The habenulae, pineal body, and posterior commissure are generally 

 included under the head of epithalamus.] 



The posterior perforated substance or postperforatum (locus perforatus posticus) 

 has been described on page 900. It marks the situation of the " interpeduncular 

 ganglion," which is small in man, but very large in rodent brains. From the 

 cells in this primitive gray lamina arise the fibre tracts already described as the 

 taenia pontis (p. 897), and often visible at the point of emergence from the gray 

 substance of the intercrural space. 



The corpora albicantia (Fig. 669), or corpora mammillaria, are tw r o symmetrical, 

 small, round, white protuberances situated side by side in the intercrural space 

 cephalad of the posterior perforated substance, at a point where the floor of the 

 third ventricle rapidly decreases in thickness to form the tuber cinereum. The 

 color of each corpus albicans is white, owing to a superficial stratum of fibres 

 derived from the fornix. Within lie three nuclear masses two medial, consti- 

 tuting the main mass, and a smaller lateral nucleus applied against the former, 

 so as to represent a crescent on cross-section. 



The fibres of the fornix terminate in the corpus albicans. From its cells arise 

 two fasciculi which have a common neurone origin. Cajal discovered, and Kolliker 

 confirmed the fact, that the axones from the medial nucleus cells bifurcate; one 

 set of limbs passes fronto-dorsad to form the fasciculus thalamomammillaris (bundle 

 of Vicq d'Azyr), which ends in the nucleus anterius of the thalamus, while the 

 other set of limbs of the primary axones passes caudad to form the fasciculus 



