ANTERIOR CEREBRAL COMMISSURE 871 



substance of the lateral wall of the third ventricle, and passes downward to the 

 base of the brain, where it appears on the exterior as the mammillary body 

 [corpus mammillare] (fig. 671). 



Some of its fibres are interrupted in the nuclei of the mammillary body, chiefly in its lateral 

 nucleus; probably most of them merely double back, forming a genu. From the mammillary 

 body the fibres are disposed in at least three ways: — (1) The greater part perhaps pass directly 

 upward and are lost in the anterior nucleus of the thalamus, where they ramify freely and term- 

 inate about its cells. These fibres form the bundle knowm as the mammillo-thalamic fasciculus, 

 or bundle of Vicq d'Azyr; (2) A portion of the fibres go to form a mammillo-mesencephalic fas- 

 ciculus (tegmento-mammillary fasciculus, mammillo-peduncular fasciculus . This begins in the 

 mammillary body and passes caudalward into the mesencephalon to terminate about ceU-bodies in 

 or in the region of, the so-called nucleus of the medial longitudinal fasciculus and posterior com- 

 missure. Fibres given by these ceU-bodies may convey impulses by way of the medial longi- 

 tudinal fasciculus or the general reticular formation to the nuclei in the mesencephalon, rhomb- 

 encephalon and perhaps into the spinal cord. Some of this portion of the fibres from the mam- 

 millary body are said to pass caudalward through the mesencephalon without interruption 

 there. (3) A portion of the fibres decussate in the superior parts of the mamillary bodies 

 and are distributed to both the thalamus and the mesencephalon of the opposite side. This 

 decussation is the supramamillary commissure. 



As seen above, the fornix as a whole is composed of longitudinally directed fibres, some of 

 which, however, cross the mid-line in the region of its body and course in the columns of the 

 opposite side. For the greater part, its fibres rise from the cells of the hippocampal gyri, but 

 it is known to contain some fibres which arise in the anterior perforated substance and sub- 

 callosal g\TUS and course through the fornix to the hippocampal gyri. 



The medial and lateral longitudinal striae upon the corpus callosum consist of olfactory 

 fibres coursing in both directions: (1) fibres arising in the parolfactory area, the subcallosal 

 gyrus and the anterior perforated substance (diagonal band of Broca) course posteriorly and 

 then inferiorly in them to the grey substance of the gryi of the hippocampus; (2) and chiefly, 

 fibres from the hippocampal gyri course in them anteriorh' and inferiorly around the rostrum 

 of the corpus caUosum, through the ventral part of the septum pellucidum, to join the fornix. 

 It is suggested that the striae, especially the medial, may be considered as a part of the fornix 

 detached upon the dorsal surface of the corpus callosum during the projection of the latter 

 between the cerebral hemispheres. The medial stria is often called the stria Lancisii. 



The anterior cerebral commissure is only in part concerned in the rhinenceph- 

 alon; it consists in greater part of commissural fibres connecting the two temporal 

 lobes. It forms one of the four commissures of the telencephalon, the other three 

 being the corpus callosum, the hippocampal commissure and the inferior cerebral 

 commissure. It is a bundle of white substance with a slightly twisted appearance, 

 which crosses the mid-line in the anterior boundary of the third ventricle be- 

 tween the lamina terminahs and the columns of the fornix (figs. 671 and 684), 

 just below the interventricular foramen (foramen of Monro). In each hemis- 

 phere its main or temporal portion passes lateralward and slightly backward 

 beneath the head of the caudate nucleus and through the anterior end of the 

 lenticular nucleus, and thence is dispersed to the grey substance of the temporal 

 lobe. 



It contains fibres both to and from the temporal lobe of each side. In addition to these 

 fibres the anterior commissure carries in its frontal side two sets of fibres belonging to the ol- 

 factory apparatus: — (1) fibres arising in the olfactory bulb of one side, which pass by way of the 

 medial olfactory striae through it to the olfactory bulb of the opposite side; (2) fibres which pass 

 through it from the medial stria (olfactory bulb) of one side to the uncus of the opposite side. 



The anterior commissure is a more primitive commissure than the corpus callosum, in that 

 it is present in the lower forms when the latter is absent, and diminishes in relative size and 

 importance as the corpus caUosum appears and increases in size. In man the appearance of the 

 anterior commissure precedes but little that of the corpus callosum. During the fifth month 

 the lamina terminahs, which then alone unites the anterior ends of the two hemispheres, develops 

 a thickening of its dorsal portion. In a part of this thickening, transverse fibres begin to appear 

 and their increase in number results in the partial separation posteriorly of the part containing 

 them from the rest of the lamina, and then follows the differentiation of this part into the anterior 

 commissure. The remainder of the thickening of the lamina continues to increase in size with 

 the increase of the hemispheres; its upper edge is directed posteriorly, and fibres begin to appear 

 in it which arise in the cortex of one side and cross over to that of the other side. These fibres 

 form the corpus callosum. 



The corpus callosum, a development of fibres in the upper, expanded portion of the lamina 

 terminahs, thus bridges over a portion of the longitudinal fissure between the hemispheres. In 

 the mean time, the fornix arises as two bimdles of fibres, one from the hippocampus of each side. 

 In the complex mechanics of the development of the cerebrum these two bundles approach each 

 other under the corpus callosum, fuse for a certain distance, and together arch the cavity of 

 the third ventricle and come to acquire their adult position. There results from these processes 

 of growth a completely enclosed space, a portion of the longitudinal fissure, the roof of which 

 is the corpus caUosum, its floor, the body of the fornix, and its lateral walls, portions of the 

 mesial surfaces of the two cerebral hemispheres. The lateral waUs of this space do not thicken 



