DIENCEPHALON 241 



HYPOTHALAMUS 



The liypothalamus as here defined inchides the ventral part of the 

 brain stem between the anterior commissure ridge and the tuber- 

 culum posterius. At its anterodorsal border the nucleus of the olfacto- 

 habenular tract might be included or assigned to the ventral thala- 

 mus. Its fibrous connections, like those of the preoptic nucleus, are 

 mainly of hypothalamic type. Posteriorly of the chiasma ridge the 

 deep sulcus hypothalamicus separates the dorsal from the ventral 

 part of the hypothalamus, and each of these parts is further sub- 

 divided. A shallow and variable sulcus hypothalamicus dorsalis sepa- 

 rates the dorsal hypothalamus again into dorsal and ventral lobes, 

 both of which are confluent with the ventral thalamus and peduncle 

 (fig. 2). These parts contain the primordium of the mamillary 

 body, but this structure is not recognizably differentiated. The ven- 

 tral hypothalamus is obscurely separable into posteroventral and 

 anterodorsal parts. All these subdivisions are more clearly seen in 

 Necturus than in Amblystoma ('35a, p. 253). 



The sulcus preopticus separates the preoptic nucleus into anterior 

 and posterior lobes. The lateral preoptic recess at the anteroventral 

 border of the chiasma ridge is a remnant of the lumen of the hollow 

 epithelial optic stalk of the embryo ('41), which persists in the endo- 

 cranial part of the optic nerve of adult Necturus ('41a, p. 494). 



The sequence of differentiation of the nervous connections of the 

 hypothalamus has been summarized elsewhere, together with a de- 

 scription of these connections in midlarval stages ('396, p. 550). The 

 adult structure and connections of the hypothalamus of Necturus 

 have been fully described ('336, '346, '41a), and those of Amblystoma 

 are essentially similar, though with considerable advance in differen- 

 tiation throughout ('42, pp. 211 ff.). None of the hypothalamic nuclei 

 described in mammals are here clearly segregated, though some of 

 them are recognizable in more dispersed arrangement. The preoptic 

 nucleus is an exception to this. It is very large, and its boundaries are 

 well defined except anterodorsally, where it merges with the nucleus 

 of the olfacto-habenular tract, and posterodorsally, where it merges 

 with the ventral part of the hypothalamus. 



AFFERENT CONNECTIONS 



The nervus terminalis, as described by McKibben ('11), has end- 

 ings distributed throughout the hypothalamus both before and be- 

 hind the chiasma ridge. The only other peripheral fibers which reach 



