34 



THE NERVOUS SYSTEM 



the posterior limb of the capsule. Owing to the crowding 

 together of the motor fibres, a very small haemorrhage may be 

 sufficient to cause complete hemiplegia, and, in addition, 

 irregularly distributed areas of sensory disturbance. Clinical 

 evidence shows that certain muscles very constantly remained 

 unaffected or only slightly affected in lesions of the internal 

 capsule. The diaphragm and other muscles of the trunk, and 



FIG. 2O. Diagram to illustrate Innervation of a Muscle by both 

 Cerebral Hemispheres. 



i. Muscle. 2. Motor nucleus. 



3. Hetero-lateral upper neurone. 



4. Homo-lateral upper neurone. 



5. Cortex. 



the muscles of the upper part of the face, are not paralysed. 

 This immunity is explained on the grounds that muscles which 

 are accustomed to act together are bilaterally represented in 

 the cortex, and their motor fibres, therefore, descend in the 

 internal capsules of both cerebral hemispheres (Broadbent's 

 law). The same explanation accounts for the relatively 

 smaller degree of paralysis in the lower limbs as compared 

 with the upper limbs. 



