CH. XLVI.] THE INTERNAL CAPSULE. 645 



matter is separated from this to form a narrow isolated strip of 

 grey matter called the claustrum (cl.). In the middle line from 

 above down are seen the great longitudinal fissure extending as 

 far as (c.c.) the corpus callosum, the band of white matter that 

 forms the great commissure between the two hemispheres ; be- 

 neath this are the lateral ventricles which communicate by the 

 foramen of Monro with the third ventricle : the fornix is indicated 

 by the letter /. Contributing to the floor of the lateral ven- 

 tricle, one next sees the optic thalamus (th.), and the tail end of 

 the nucleus caudatus (n.c.) ; the section being taken somewhat 

 posteriorly. The nucleus lenticularis is marked id. ; and the 

 band of white fibres passing up between it and the thalamus is 

 called the internal capsule (c.t.) ; the narrow piece of white matter 

 between the claustrum and the lenticular nucleus is called the 

 external capsule. 



For the student of medicine the internal capsule is one of the 

 most important parts of the brain. Into it are continued up the 

 fibres which we have previously traced as far as the cms cerebri ; 

 the motor-fibres of the crusta are continued into the anterior two- 

 thirds of its posterior limb (i.e. behind the genu * in fig. 479); 

 the sensory fibres of the tegmentum into the posterior third of 

 this limb. When these fibres get beyond the narrow pass between 

 the basal ganglia, they spread out in a fan-like manner and are 

 distributed to the grey cortex; the motor-fibres going to the motor 

 convolutions around the fissure of Rolando; the sensory fibres to the 

 same convolutions and also to others behind these which are asso- 

 ciated with special sensations. The name corona radiata is applied 

 to the fan-like spreading of the fibres ; the fibres as they pass 

 through the handle of the fan, or internal capsule, communicate 

 with the nerve-cells of the grey matter of the basal ganglia ; the 

 pyramidal fibres on their way down to the medulla and cord from 

 the motor areas of the brain send off collaterals or side branches 

 which arborise around the cells of the corpus striatum, and to 

 a lesser degree around those of the optic thalamus ; the axis- 

 cylinder processes of these cells pass out to join the pyramidal 

 tract on its downward course. The sensory fibres on their way 

 up may pass straight on to the cortex, but the majority, especially 

 those in the fillet, terminate by arborising round the cells of the 

 optic thalamus, and in the subthalamic area. This, in fact, is 

 another cell-station or position of relay : the fibres passing out from 

 the cells of the thalamus continue the impulse on to the cortex. 



The importance of the internal capsule is rendered evident 

 when one considers the blood supply of these parts; at the 

 anterior and posterior perforated spots, numerous small blood-vessels 



