82 Anatomy of the Nervous System 



by the commissure of Meynert (commissura siipraoptica 

 dorsalis). The latter is better developed in the rat than in 

 the human species and forms a conspicuous decussation 

 postero-dorsal to the optic fibres. It is composed of fine 

 fibres, which may be observed running along the medial aspect 

 of the optic tract (PI. XMIL). These fibres are probably 

 telencephalic, but their exact relations are still uncertain. 



A small, aberrant bundle of optic fibres is separated from 

 the rest by the commissures of v. Gudden and Meynert, 

 crossing in the most posterior part of the chiasma. They 

 gradually penetrate these commissures, however, and pass 

 through them to join the main optic tract, a few ending in 

 the subthalamic nucleus. 



There may perhaps be included with the geniculate bodies 

 under the heading metathalamus^ the posterior nucleus of 

 the thalamus {nucleus posterior thalami, nucleus praebige- 

 minalis), which is related chiefly to the midbrain and has no 

 ascending cortical connections. It is a rather large group of 

 cells lying medial to the medial geniculate body and the 

 dorsal part of the lateral nucleus at the posterior end of the 

 thalamus (Pis. XVI., XYIL). It receives a group of collaterals 

 from the medial lemniscus. 



The remainder of the thalamus falls into three chief 

 divisions, the anterior, medial, and ventro -lateral nuclei. 



The anterior nucleus {nucleus anterior thalami, nucleus 

 dorsalis thalami) is wedged between the rostral ends of the 

 other two groups and extends farther anteriorly than either 

 of them (PI. XX.). The bundle of \'icq d'Azyr has already 

 been traced to this nucleus, which receives through it im- 

 pulses from olfactory centres. Thus it is not surprising to 

 find that the size of the anterior nucleus of the thalamus 

 varies in different animals with the degree of development of 

 the olfactory system, though not related to it alone. Hence 



^According to Kappers. Other writers regard the posterior nucleus 

 as a part of the lateral nucleus. 



