532 ELEMENTARY PHYSIOLOGY less. 



volution (see Fig. 170). The disorder is, from its nature, 

 known as aphasia («, privative of, (fida-is, speech) and 

 may take several forms ranging from complete inability 

 to speak at all to an inability to utter certain words, and 

 hence to speak coherently. This centre for speech is 

 curiously, and unlike most of the other centres, unilateral, 

 being situated on the left side of the brain in ordinary 

 right-handed persons and in the corresponding part of 

 the right side of the brain in those who are left-handed. 



We need say no more in proof of the connection of 

 part of the right side of the brain in those who are left- 

 handed. Speech however is a very complicated operation 

 involving a number of mental processes such as the 

 thinking of the words to say, and the command of 

 muscular movements for their production. Broca's area 

 is probably involved in the latter and is by no means the 

 only portion of the brain involved in speech. 



The portion of the brain concerned with tactile sen- 

 siition is close to the motor areas, being situated just 

 beliind, instead of just in fnnit of the fissure of Rolando. 

 The situations in which, so far as we know, afferent 

 impulses are converted into tliose altered states of con- 

 sciousness to which we give the name of sensations are 

 shown on Figs. 170 and 171. 



The Internal Capsule. — The brain, as we have 

 previously said, may be regarded as built up round a very 

 peculiarly shaped central canal by means of thickenings 

 of the walls of that canal due to the development of 

 masses of nerve fibres and deposits of grey matter. We 

 have described the position of the most conspicuous of 

 these deposits and have incidentally referred to many of 

 the more important inter-connections of the chief parts of 

 which the brain as a whole consists. Moreover, we have 

 referred in some detail to the nature of the connection of 

 the spinal cord with the biain by means of very definite 

 " tracts " of fibres. Among these, particular stress was 

 laid on that tract which is known as the (crossed) 

 pyramidal tract, and it was pointed out (p. 492) that the 



