THE INTERNAL CAPSULE 35 



The anterior limb of the internal capsule is occupied chiefly 

 by fibres which connect the cortex of the frontal lobe with 

 the grey matter of the pons, and nothing is known concerning 

 their function. 



The posterior or retro-lenticular part of the posterior limb 

 contains (i) Sensory fibres from the opposite side of the 

 body; (2) the acoustic radiation, which extends from the 

 lower acoustic centres to the cortex of the superior temporal 

 gyrus; (3) the optic radiation, which lies behind (i) and 

 (2), extending from the lower visual centres (p. 51) to the 

 occipital cortex. It is probable that the fibres of the other 

 special senses, namely, taste and smell, also pass through 

 the posterior third of the posterior limb of the internal 

 capsule. 



The manner in which the fibres of the internal capsule 

 converge below and spread out above is well shown in 

 Fig. 21, which represents a dissection carried out from the 

 lateral aspect of the brain. The cortical areas which overlap 

 the island (of Reil), the island itself, the claustrum, the 

 external capsule and the lentiform nucleus have all been 

 removed. The connexions of the capsule with the different 

 parts of the cortex are clearly indicated, and it is evident that 

 the lower the position of a lesion, the more complicated and 

 widespread will be its effects. 



A lesion of the internal capsule may be due to pressure, 

 from tumour growth, haemorrhage, etc., or to anaemia, following 

 occlusion of the vessels of supply. When of large extent, it 

 is accompanied by complete hemiplegia, hemi-anaesthesia, loss 

 of muscle and joint sense, and deafness, all on the opposite 

 side, and homonymous hemianopia affecting the corresponding 

 sides of the retinae. A smaller haemorrhage may produce 

 complete hemiplegia, with irregular sensory phenomena which 

 are never confined to one limb. 



A lesion of the postero-lateral part of the thalamus causes 

 hemi-anaesthesia of the whole of the opposite half of the body. 

 The condition is usually associated with disturbances of vision 



