DiENCEPHALON 73 



white fibres and composed of two distinct nuclei. These 

 nuclei are a larger, lateral mass of rather scattered cells with 

 many myelinated fibres and a smaller, medial group of many, 

 closely-packed, smaller cells with few white fibres. Anteriorly, 

 the medial nucleus di\'ides into dorsal and ventral portions, 

 in the rat, while posteriorly it is continuous with that of the 

 other side through the poorly-developed bed nucleus of the 

 habenular commissure. The dorsal layer of white matter is 

 a definite band composed of compact bundles of longitudinal 

 fibres, the stria medullaris thalami, of which the medial edge 

 projects slightly into the membranous roof as the taenia 

 thalami. The latter forms a small but sharp ridge marking 

 the dorsal extremity of the wall of the third ventricle. It is 

 composed of olfactory fibres of various kinds (see below), all 

 of which probably end in the nuclei habenulae, so that the 

 stria medullaris diminishes in size as it passes posteriorly. 

 Many of its fibres, nevertheless, are still to be seen just 

 behind the posterior end of the nucleus. These cross the 

 middle line directly dorsal to the posterior commissure to 

 terminate in the habenular nucleus of the other side, thereby 

 constituting the habenular commissure {commissura habenu- 

 larum, superior commissure) (Pis. XML, XXM., drawn but 

 not named in PI. III.), which is small in the rat. Traced 

 anteriorly, the stria medullaris is seen to turn ventrally at 

 the anterior end of the thalamus and to spread out in the 

 ventro-medial part of the hemisphere, where it meets the 

 longitudinal fibres of the diffuse medial forebrain bundle. 



A summary of the composition of the stria medullaris of the albino 

 rat has been given by Herrick,^ who lists eight component tracts, as 

 follows: . 



(1) Tractus olfacto-habenularis medialis — composed of fibres from the 

 praeoptic nucleus and surrounding gray matter. There are anterior and 

 posterior parts of this tract running respecti\'ely in front of and behind the 



^"The conclusions as I published them should be accepted with some 

 reserve, though I believe them to be substantially correct". (Herrick^ 

 personal communication.) 



