222 THE NERVOUS SYSTEM 



THE HYPOTHALAMUS 



The hypothalamus consists of three parts: (1) the pars optica hypothalami, 

 which belongs to the telencephalon, (2) the pars mamillaris hypothalami, and 

 (3) the subthalamus. 



The pars mamillaris hypothalami includes the corpora mamillaria, tuber 

 cinereum, infundibulum, and hypophysis. The mammillary bodies are a pair of 

 small spheric masses of gray matter, situated close together in the interpedun- 

 cular space rostral to the posterior perforated substance (Figs. 86, 158, 159). 

 Each is enclosed in a white capsule and projects as a rounded white eminence 

 at the base of the brain (Fig. 156). In the sheep's brain the two are fused to- 

 gether into a single eminence (Fig. 83). Each mammillary body is composed 

 of two nuclear masses: a large medial group of small cells and a smaller lateral 

 collection of large cells. The white capsule is formed by fibers from the hippo- 

 campus, which sweep in a broad curve around the thalamus, forming a bundle 

 known as thefornix (Figs. 204, 205). This descends in front of the interventric- 

 ular foramen and reaches the mammillary body, within, which a large part of 

 these fibers end. From the dorsal aspect of the medial nucleus springs a stout 

 fascicle, which runs dorsally, to end in the anterior nucleus of the thalamus, and 

 is known as the mammillothalamic tract or bundle of Vicq d'Azyr (Figs. 156, 204, 

 205). A short distance from the mammillary body there branches off from this 

 tract another, the mammillotegmental tract of Gudden, which runs caudally in 

 the tegmentum of the mesencephalon and probably ends in the dorsal tegmental 

 ganglion. The lateral nuclear mass is also connected with the tegmentum by 

 way of the peduncle of the mammillary body (Fig. 211). 



The tuber cinereum, as seen -from the ventral surface of the brain (Figs. 

 83, 86), is a slightly elevated gray area rostral to the mammillary bodies. It is 

 one of the olfactory centers. To it there is attached the funnel-shaped stalk 

 of the hypophysis, known as the infundibulum. The hypophysis is a small 

 gland of internal secretion, which is not composed of nervous tissue and which 

 interests us here only because its posterior portion is developed as an outpock- 

 eting of the ventral wall of the diencephalon, to which it remains attached by 

 the infundibulum. A detailed account of this structure may be found in the 

 papers by Tilney (1911 and 1913) listed in the Bibliography at the end of this 

 volume. 



The subthalamus is situated between the thalamus and the tegmentum of 

 the mesencephalon and forms a zone of transition between these two struc- 

 tures (Figs. 156, 157). The long sensory tracts of the tegmentum run through 



