9^4 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



a better term. The medial part of the tuber contains 

 two rather large groups, the dorsomedial and the 

 ventromedial nuclei. The dorsomedial is rather \ague 

 and contains a mixture of neurons most of which are 

 rather small. The ventromedial nucleus is quite 

 conspicuous in carnivores and the lower primates. It 

 is an oval mass of small cells which are compactly 

 grouped in lower forms but which in man have a 

 much more diffuse arrangement. Ventrally, the so- 

 called arcuate nucleus lies at the lateral edges of the 

 ventral extremity of the third ventricle, near the 

 median eminence. Extending dorsally along the walls 

 of the ventricle in this region are thin layers of small 

 neurons, many of them poorly differentiated, which 

 are grouped as a periventricular nuclear system. In 

 the posterior part of the tuberal region and extending 

 to the most posterior portion of the hypothalamus is 

 the posterior hypothalamic nucleus or area. This lies 

 between the two converging mammillothalamic 

 tracts. It is very similar in structure to the lateral 

 hypothalamic region, being a melange of large and 

 small cells. This region is important because of its 

 contribution to descending hypothalamic connections 

 and because in experimental work lesions in this 

 region epitomize some of the effects of more extensive 

 lesions in the anterior areas. 



c) The third portion of the h\pothalamus is the 

 mammillary area. This is marked principally by the 

 presence of the bulging mammillary bodies which 

 contain a complex of nuclei. A large oval medial 

 nucleus is separated from a lateral nucleus, of larger 

 and darkly-staining cells, by an intercalated nucleus. 

 Anterior to the mammillary nuclei is the premamil- 

 lary region which, in lower forms at least, is marked 

 by another complex of small nuclear groups, not 

 very conspicuous in man. They appear to receive 

 fibers ascending from the tegmentum in the mamil- 

 lary peduncle. 



AFFERENT CONNECTIONS. A great deal has been written 

 concerning the fiber connections of the hypothalamus, 

 and there is reasonably good evidence for the follow- 

 ing afferent connections of the primate hypothalamus. 

 a) The inedial forebrain bundle, which contains 

 olfactory, parolfactory, septal and striohypothalamic 

 fibers, may be mentioned first. The septohypo- 

 thalamic fibers probably relay impulses from the 

 frontal lobe of the cerebral cortex and perhaps also 

 impulses originating in the rhinencephalic regions. 

 h) The thalamohypothalamic fibers, which arise 

 chiefly from the medial and mid-line thalamic nuclei 

 and run principally by way of the peri\-entricular 



system, may be important for relaying somatic and 

 visceral sensory impulses to the hypothalamus. These 

 connections probably also set up the major afferent 

 connections between the neopallial cortex and the 

 hypothalamus, for the dorsomedial thalamic nucleus 

 shows degeneration after prefrontal lobotomy and 

 also after hypothalamic lesions (164). Although direct 

 connections from the orbitofrontal regions of the 

 hemisphere to the \entromedial nucleus of the hypo- 

 thalamus ha\e ijeen described, more recent work 

 with modern degeneration techniques has not sup- 

 ported this finding (9). cj The fornix is a prominent 

 bundle which arises in the hippocampus, terminates 

 in the mammillary nuclei and probably also sends 

 some fibers into other hypothalamic nuclei (126). 

 dj The stria terminalis, which arises from the amyg- 

 dala, appears to ha\e connections with the preoptic 

 regions, perhaps with the septal area, and has a 

 rather diffuse distribution in the hypothalamus. The 

 chief evidence for these connections is based on com- 

 parative studies of lower animal forms, e) Pallido- 

 hypothalamic fibers constitute a well defined bundle 

 which arises in the lentiform nucleus and appears 

 to terminate in the ventromedial hypothalamic 

 nucleus. Other connections with the globus pallidus 

 by way of the ansa lenticularis are also possible. 

 /) Subthalamoh\ pothalamic connections are probably 

 mediated by fiijers from the nucleus subthalamicus, 

 chiefly crossed, g) The mammillar\' peduncle ri.ses in 

 the me.sencephalDn and ends in the lateral mammillary 

 nucleus. Its existence in man is questionable. //) \'ago- 

 supraoptic connections, which have been demon- 

 strated physiologically (39, 145), presumably arise 

 in the nucleus of the solitary tract, but whether they 

 set up direct or indirect connections with the hypo- 

 thalamus is not known. 



EFFERENT CONNECTIONS. Efferent connections of the 

 hypothalamus may be summarized as follows, a) 

 Hypothalamothalamic fibers include the mammillo- 

 thalamic tract which extends from the mammillary 

 nuclei to the anterior thalamic nuclei which in turn 

 are connected with the anterior part of the limbic 

 cortex (gyrus cinguli), and periventricular fibers, 

 presumably connecting with the dorsomedial thalamic 

 nucleus which in turn connects with the frontal 

 cortex, h) The mammillotegmental tract descends 

 to the deep tegmental nucleus of the lower brain 

 stem. () The perixentricular system and the dorsal 

 longitudinal fasciculus are important for conduction 

 of descending impulses. These fibers may arise 

 throughout the hypothalamus, but the posterior 



