THE MAMMALIAN NERVOUS SYSTEM 117 



of the thalamus (tuherculum 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 colli- 

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

 the floor of the aqueduct of Sylvius, where they effect connec- 

 tions 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 corre- 

 lation 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. 

 They run just in front of the optic radiations and end in the 

 temporal lobe of the cortex. 



149. Somesthetic radiations. Carefully tease the lateral and 

 medial lemniscus fibers from the midbrain region, into which 

 they have been traced (Sections 106 and 108), upward into 

 the thalamus. Their terminal nuclei comprise the lateral and 

 ventral nuclei of the thalamus, lying internally of the pulvinar. 

 From these nuclei strands of fibers can be torn upward toward 

 the cortex into the internal capsule. These are the somatic 

 sensory radiations destined for the gyms centralis posterior. 



150. Pyramidal tract. Next tear the pyramidal tract up- 

 ward from the midbrain floor along the cerebral peduncle into 



