22O THE NERVOUS SYSTEM 



THE METATHALAMUS 



The metathalamus is composed of two small protuberances, the geniculate 

 bodies, which, having been displaced by the excessive development of the 

 thalamus, are situated upon the dorsolateral surface of the rostral end of the 

 mesencaphalon (Figs. 87-89, 154, 161). The lateral geniculate body is an oval 

 swelling in the course of the optic tract. Its connections will be more fully 

 considered in connection with the discussion of the course of the visual impulses. 

 The medial geniculate body is overhung by the pulvinar, from which it is separated 

 by a deep sulcus. It receives fibers by way of the inferior quadrigeminal bra- 

 chium from the lateral lemniscus, which we have learned to know as the central 

 auditory path from the cochlear nuclei. From it fibers run to the auditory 

 area of the cerebral cortex (the thalamotemporal or acoustic radiation). 



THE EPITHALAMUS 



The epithalamus includes the pineal body, stria medullaris, and habenular 

 trigone. The latter is a small triangular depressed area located on the dorso- 

 medial aspect of the thalamus rostral to the pineal body (Fig. 158) . In the sheep, 

 as in most other mammals, it is much larger than in man and bulges both dor- 

 sally and medially beyond the surface of the thalamus (Figs. 91, 159). It marks 

 the position of a nuclear mass, called the habenular ganglion, which receives fibers 

 from the stria medullaris, a fascicle which runs along the border between the 

 dorsal and medial surfaces of the thalamus subjacent to the taenia thalami 

 (Figs. 154, 155). The stria medullaris takes origin from the anterior perforated 

 substance and other olfactory centers on the basal surface of the cerebral hemi- 

 sphere and, partially encircling the thalamus, reaches the habenular ganglion, 

 in which it ends. (See p. 281.) Not all of the fibers terminate on the same 

 side; some cross to the ganglion of the opposite side, forming a transverse bundle 

 of myelinated fibers which joins the caudal end of the two ganglia together and 

 is known as the habenular commissure. From the cells in this ganglion arises 

 a bundle of fibers, known as the fasciculus retroflexus of Meynert or the tractus 

 habenulopeduncularis. This bundle of fibers is directed ventralward and at 

 the same time caudally along the medial side of the red nucleus toward the 

 base of the brain, where it crosses to the opposite side and ends in the inter- 

 peduncular ganglion (Fig. 189). The stria medullaris, habenular ganglion, 

 and fasciculus retroflexus are all parts of an arc for olfactory reflexes, as indi- 

 cated in Fig. 211. According to Edinger (1911) the cells, from which the stria 

 medullaris arises, are intimately related to a bundle of ascending fibers from 



