THE CENTRAL NERVOUS SYSTEM. 93 



with knowledge, intelligence, and, still further, with the faculties of 

 inhibition and voluntary attention, which find their highest development 

 in man. 



SPEECH AND APHASIA. 



The pre-eminence of man is intimately related with the power of 

 speech or the use of words as symbols to awaken the memory of past 

 stimuli. The production of spoken or written words is of course only 

 a specialised use of the muscular system ; but for the appreciation of 

 language the existence of word-hearing and word-seeing centres has 

 sometimes been assumed. It is found that aphasia or loss of the 

 power of speech may take several forms. Thus there may be motor 

 aphasia, in which the power of forming words is lost, or sensory 

 aphasia, in which there is inability to comprehend spoken or written 

 language. 



Motor aphasia was formerly ascribed to a lesion of the third left 

 frontal (Broca's) convolution, but it has been ascertained that the 

 actual cause of the aphasia in such a case is destruction of the sub- 

 cortical fibres in the neighbourhood of the lenticular nucleus, along 

 with some degree of sensory aphasia. The term anarthria is applied 

 to the loss of power of articulate speech due to a lesion of the sub- 

 cortical motor fibres. Sensory aphasia may take the form of word- 

 blindness or word-deafness. In word-blindness, vision may be perfect 

 and the words on a printed page ma}^ be distinctly seen, but they are as 

 meaningless as if they were in an unknown language, and there is no 

 power of associating the written symbol with past stimuli. Similarly 

 in word-deafness, hearing may be perfect, but spoken words are un- 

 intelligible sounds. Anarthria may occur without any loss of intelli- 

 gence, but sensory aphasia is always accompanied by some degree of 

 mental deficiency, especially in the case of word-deafness. 



THE THALAMUS AND INTERNAL CAPSULE. 



The thalamus forms the lateral boundary of the third ventricle. 

 It is a large ganglioriic mass and receives the terminations of the fibres 

 of the fillet. The outgoing fibres from the thalamus are distributed to 

 all parts of the cortex of the cerebral hemispheres, and cortico-thalamic 

 fibres also run from the cortex to the thalamus. On the lateral aspect 

 of the thalamus is the internal capsule, to and from which fibres 

 radiate from and to all parts of the cortex. In a horizontal section, the 

 internal capsule is seen to consist of a short anterior limb pointing 

 outwards and forwards, and a longer posterior limb pointing outwards 

 and backwards ; the junction of the two limbs is termed the genu. 



