906 THE NERVE SYSTEM 



This surface is of a whitish color owing to a thin layer of white fibres, the stratum 

 zonale. A faint oblique groove crosses this surface in a caudolateral direction, 

 corresponding to the ectal edge of the fornix. Laterad it is demarcated from the 

 caudate nucleus by a groove which is occupied by a slender band of fibres and the 

 striatal vein, called the taenia semicircularis or stria terminalis. The surface is 

 not of even contour throughout, usually showing three eminences (in addition 

 to the pulvinar) corresponding to the main nuclear aggregations within the thal- 

 amus viz. : 



Tuberculum anterius. 



Tuberculum medialis. 



Tuberculum lateralis. 



The tuberculum anterius forms a marked bulging frontal extremity, which helps 

 to form the boundary of the foramen of Monro or aperture of communication 

 between lateral and third ventricles. 



The caudal extremity of the thalamus is a prominent bolster-like projection 

 which overhangs the brachia of the corpora quadrigemina and is called the pul- 

 vinar. A smaller oval prominence, situated ventrolaterad of the pulvinar. is 

 termed the external geniculate body or pregeniculum (corpus geniculatum laterale) 

 a partial end station for the optic tract. 



The lateral surface of the thalamus is in contact with the internal capsule that 

 great concentration of fibre tracts coursing to and from cerebral centres and 

 forming the crusta below. To this white stratum the thalamus itself contributes 

 fibres destined to reach the cortex, and in turn it receives fibres from the cortex. 

 These thalamocortical and corticothalamic sets of fibres constitute the thalamic 

 radiation, forming a more or less distinct reticulated capsular zone (external 

 medullary lamina; stratum reticulatum) for the thalamus. 



The ventral or inferior surface is continuous with the subthalamic tegmental 

 substance and with the central gray substance of the third ventricle lining its sides 

 and floor. 



Internal Structure of the Thalamus. The thalamus is composed of gray substance, 

 with large multipolar cells, which is subdivided into a number of distinct nuclei; 

 twenty such have been described; three are universally recognized. They are 

 separated from each other by a white layer (lamina medullaris interna) which 

 runs parallel to the wall of the third ventricle for its greater length; caudally 

 it runs mesad, overlapped by the ectal nucleus, and numerous sub-laminae run 

 into it. Frontad the internal medullary lamina subdivides into two branches, 

 thus permitting the intrusion of the nucleus anterius between the two main nuclei 

 (medial and lateral). 



The nucleus anterius lies f rontodorsad ; in it terminate the axones of the fasciculus 

 thalamomammillaris (fasciculus albicantiothalami bundle of Vicq d'Azyr). The 

 nucleus medialis is lined mesad by the central gray of the third ventricle and is 

 usually fused with its fellow of the opposite side (middle commissure). A special 

 spheroidal cell cluster in this nucleus is called the centrum medianum (Luys). 

 The nucleus lateralis is the largest of the three, extending the entire length of the 

 thalamus and including the pulvinar. A special semilunar cluster of cells in the 

 ventral portion of this nucleus is called the nucleus semilunaris (Flechsig). 



The Connections of the Thalamus. The thalamus is a ganglion interposed 

 between the sensor tracts in the tegmentum and the cerebral cortex, as well as 

 an important link in the optic path. It also gives rise to motor tracts concerned 

 with instinctive movements of an emotional nature. It is a relay station for 

 the various tracts which convey sensations of touch, temperature, and pain from 

 the body, extremities, head and neck, of muscle sense, and of the special senses. 

 It transmits these impulses to, and, reciprocally, receives impulses from the 



