CH. XLVI.] THE INTERNAL CAPSULE 655 



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

 posteriorly. The nucleus lenticularis is marked nl. ; and the band of 

 white fibres passing up between it and the thalamus is called the 

 internal capsule (c.i.) ; 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 crus 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. 477) ; 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 coming down from the motor con- 

 volutions around the fissure of Eolando ; the sensory fibres going to 

 the same convolutions and also to others behind these which are 

 associated 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 enter for the 

 supply of the basal ganglia, and these are liable to become diseased, 

 and if they rupture, a condition called apoplexy is the result ; if the 

 haemorrhage is excessive, death may occur almost immediately ; but 

 if the patient recovers, a condition of more or less permanent paralysis 

 remains behind ; and a very large amount of paralysis results from a 

 comparatively limited lesion, because so many fibres are congregated 

 together in this narrow isthmus of white matter. If the haemorrhage 

 is in the anterior part of the posterior limb, motor paralysis of the 

 opposite side of the body (hemiplegia) will be the most marked 

 symptom. If the haemorrhage occurs in the posterior part, sensory 

 paralysis of the opposite side of the body will be the most marked 



