THE DIENCEPHALON 



845 



oidal sulcus. Each thalamus is separated laterally from the caudate nucleus of 

 the telencephalon, by a Hnear continuation of the white substance below, known 

 as the stria terminalis thalami (taenia semicircularis). Like the quadrigemina, 

 each thalamus is covered by a thin capsule of white substance, the stratum 

 zonale. The average length of the thalamus is about 38 mm., and its width about 

 14 mm.; its inferior extremity is directed obliquely lateralward. The dorsal 

 surface usually shows four eminences, indicating the position of the so-called 

 nuclei of the thalamus within. These are the anterior nucleus or anterior tubercle, 

 the medial nucleus or tubercle, the lateral nucleus, and the yulvinar, the tubercle of 

 the posterior extremity. The pulvinar of the human brain is peculiar in the fact 

 that it is so developed as to project inferiorly and slightly overhang the level of 

 the quadrigeminate bodies. The projecting portion assumes relations with the 

 optic tract and the metathalamus. 



Fig. 



666. — Dissection of Brain showing Metathalamus and Pulvinar with Adjacent 



Structures. 



Caudate nucleus 



Stria terminalis of thalamus 



Pulvinar 



Optic tract 



Inferior quadrigem 

 nate body 



Medial geniculate body 

 Lateral geniculate body 



Mammillary body 



Optic tract 



Lateral stria of olfactory 

 tract 



Olfactory bulb 



Insula (central lobe) 

 Tail of caudate nucleus 



Both the structures of the metathalamus, the lateral and medial geniculate 

 bodies, are connected with the optic tract, but it is thought that actual visual 

 axones terminate only in the lateral genticulate boclJ^ As the optic tract curves 

 around the cerebral peduncle it divides into two main roots. The lateral gen- 

 iculate body receives a small portion of the fibres of the lateral root of the optic 

 tract; the remainder pass under this body and enter the pulvinar of the thalamus. 

 The medial genicidate body is connected with the medial root of the optic tract, 

 which root consists largety, not of retinal fibres, as does the lateral root, but of the 

 fibres forming Gudden's commissure (the inferior cerebral commissure). The 

 retinal fibres contained in the medial root pass to terminate in the superior 

 quadrigeminate bodies. 



Of the epithalamus, the epiphysis (pineal body, conarium) is the most con- 

 spicuous external feature. This is an unpaired, cone-shaped structure, about 7 

 mm. long and 4 mm. broad, which also projects upon the mesencephalon so that 

 its body rests in the groove between the superior quadrigeminate bodies. Its 

 stem is attached in the mid-line at the posterior extremity of the third ventricle, 

 and therefore just above the posterior commissure of the cerebrum (fig. 658). 

 It is covered by pia mater, and is involved in a continuation of the tela chorioidea 



