THE THALAMUS 



881 



surface is concave, conforming to the convexity of the putamen. The sheet of 

 white substance intervening between it and the putamen is known as the external 

 capsule. Its lateral surface shows ridges or projections in section which conform 

 to the neighbouring gyri of the insula, and it is spread through an area which quite 

 closely coincides with that of the inusla. Below and anteriorly it becomes con- 

 tinuous with the cortex of the anterior perforated substance and with the lenticu- 

 lar nucleus at the region of the junction of these. Above and posteriorly it gradu- 

 alty becomes thinner, and finally disappears in the white substance about it. 

 In origin it is thought to be a detached portion of the cortical grey substance of 

 the insula. 



The amygdaloid nucleus [nucleus amygdalae] is represented by the amygda- 

 loid tubercle, which has already been described in the extremity of the inferior 

 cornu of the lateral ventricle (figs. 666 and 691). It is an almond-shaped mass of 

 cells joined to the tail of the caudate nucleus, continuous above with the putamen 

 and anteriorly continuous with the cortex of the hippocampal gyrus. 



Fig. 693. — Coronal Section of Telencephalon through the Anterior Commissure, 

 Optic Chiasm a, and Trunk of Corpus Callosum. (After Toldt, "Atlas of Human 

 Anatomy," Rebman, London and Xew York.) 



Caudate nucleus 

 (head) 



Internal capsule 

 (.frontal portion) 



,- Longitudinal fissure 

 Corpus callosum 



_ Anterior cornu 

 s j^'' of lateral ven- 

 3,,-''g tricle 



^^sg Chorioid plexus 

 jO^^r'' of lateral ven- 

 -f^^^' tricle 



Claustrum --; \ (' ' 

 Vena terminalis ''Ns^;tv ^ >''' 



Interventricular 

 foramen(Monroi) 



Anterior perfor- 

 ated substance 



Uncus 



^.Lateral fissure 

 (Sylvii) 



~'"Gyri of insula 

 ~-Optic recess 



Optic tract 

 Optic chiasma 



Inferior commissure (Guddeni) 



The chief connections of the amygdaloid nucleus by way of the stria terminalis of the 

 thalamus are noted above under the description of the posterior division of the rhinencephalon. 

 The amygdaloid nucleus, like the claustrum, is thought to represent a detached portion of the 

 cortex, it being detached from the uncus. Considering this and its chief connections, it, with 

 the stria terminaUs of the thalamus, are concerned in the central portion of the olfactory 

 apparatus. 



The thalamus and hypothalamus. — The external features of these portions of 

 the prosencephalon have been described in their natural place, but inasmuch as 

 they contain the chief relays between the telencephalon and the divisions of the 

 nervous system caudal to the prosencephalon, the consideration of their internal 

 structure has been deferred till now. The principal grey masses to be considered 

 are the thalamus and the hypothalamic nucleus. The structures comprising the 

 metathalamus and epithalamus have already been mentioned in their relations 

 with the mesencephalon and the optic and auditory apparatus. 



The thalamus has upon its upper surface, under its ependyma, a thin stratum 

 zonale of white substance, derived in part from the incoming fibres and in part 

 from its own cells. Its oblique lateral surface conforms to the medial surface of the 

 internal capsule; its vertical medial surface forms the lateral wall of the third 

 ventricle, and below it is continuous into the hypothalamic (tegmental) region. 



56 



