THE MAMMALIAN NERVOUS SYSTEM 113 



nucleus. The lentiform nucleus (Herrick ('15), Fig. 45) will 

 now be visible on the lateral aspect of the specimen, perhaps 

 still covered superficially by the fibers of the external capsule. 



145. Internal capsule. — The broken ends of the internal 

 capsule fibers will now be seen between the caudate nucleus 

 and the lentiform nucleus of the corpus striatum and between 

 the lentiform nucleus and the thalamus. Examine carefully 

 the relations of the internal capsule to the three gray masses. 

 See the list of references at the end of Section 139. 



146. Nucleus anterior thalami. — Locate the anterior nu- 

 cleus of the thalamus (also called nucleus dorsalis) which 

 forms a well-defined eminence at the anterior end of the dorsal 

 aspect of the thalamus (tuberculum anterius thalami), and into 

 which the tractus mamillo-thalamicus (Vicq d'Azyr's bundle) 

 has been traced (Section 125 (7)) . This is a part of the primitive 

 thalamus which, so far as known, has no direct cortical con- 

 nections; but fibers can be traced by careful teasing directly 

 forward into the head of the caudate nucleus. 



147. Optic connections.- — Trace the optic tract from the optic 

 chiasma to its endings in the colliculus superior (optic tectum) 

 on the one hand and to the pulvinar and lateral geniculate body 

 on the other. The tectum opticum is a mesencephalic center 

 for the unconscious reflex movements of accommodation of the 

 eyes. By carefully teasing away the gray mass of the colliculus 

 superior, fibers can be seen passing down to the region of the 

 floor of the aqueduct of Sylvius, where they effect connections 

 with the nuclei of the III and IV nerves and with the fasciculus 

 longitudinalis medialis. This fasciculus (which has already been 

 dissected out — see Section 109) is a general correlation tract 

 for the eye-muscle nerves and for all visual reflexes. Dissect 

 the optic radiations from the pulvinar to the occipital pole of 

 the cerebral hemisphere. These fibers swing outward, then 

 dorsalward and backward into the cuneus, passing up behind 

 the internal capsule fibers. 



148. Auditory connections. — The auditory path has already 

 been traced (Section 106) by way of the lateral lemniscus to its 

 thalamic nucleus, the medial geniculate body. Remove the 

 optic tract and pulvinar carefully. Then tease the fibers from 

 the medial geniculate body upward into the internal capsule. 



