78 Anatomy of the Nervous System 



and its structure and connections have not yet been thoroughly 

 elucidated. The outstanding structures in it which are usually 

 described are as follows. 



The subthalamic nucleus {corpus subthalamic urn, body or 

 nucleus of Luys) is particularly large in primates. It appears 

 in the human brain as a rather conspicuous, lens-shaped 

 nucleus lying immediately dorsal to the pes pedunculi. In 

 the rat, however, it is not clearly marked off from surrounding 

 structures in Weigert preparations. It has the general form 

 of a flattened mass of numerous rather large cells, penetrated 

 by many medullated fibres and occupying about the same 

 position as in man (Pis. XVII., XVIII.). The centre is marked 

 in the figures according to the description of Cajal and a com- 

 parison of Weigert and Nissl sections of the rat brain with 

 Weigert sections of human material. This mass appears to 

 be the nucleus proprius pedunculi cerebri as identified by 

 Winkler and Potter,^ who indicate a more diffuse group of 

 cells dorso-lateral to this as corpus subthalamicum. 



Dorsal to this region is the zona incerta (Pis. XVI I. -XIX.), 

 which in man lies between two distinct layers of white fibres 

 belonging, like the capsule of the subthalamic nucleus, to the 

 ansa lenticularis. In the rat, the fibres form a diffuse net 

 among the small cells of the region, which thus lacks, like 

 the subthalamic nucleus, the clearly marked boundaries in 

 Weigert sections which are seen in the human brain. The 

 ansa lenticularis probably contains both ascending and 

 descending fibres between the corpus striatum of the cerebral 

 hemisphere and the corpus subthalamicum and neighbouring 

 gray matter." Anteriorly this area continues beyond the 



^An Anatomical (juide to Experimental Researches on the Rabbit's 

 Brain. Amsterdam, 1911. 



^Morgan (Anat. Rec, vol. 29, p. 369) has recently called attention 

 to a tract in man running from the ansa lenticularis dorsally, medial to 

 the red nucleus, close to the mamillo-tegmental tract, and taking up a 

 position immediately lateral to the medial longitudinal bundle. While 



