556 TEXT-BOOK OF PHYSIOLOGY. 



It is quite probable that with the growth of the brain in size and complexity, 

 the motor area has come to occupy a position somewhat farther forward in 

 the human brain than in the monkey brain. 



This general area is also capable of subdivision into areas of variable size, 

 in which the movements of the face and associated structures, the head and 

 eyes, the arm, trunk, and leg, are represented. (Fig. 254.) The sequence 

 of their representation from below upward is similar to that observed in the 

 monkey and chimpanzee. In each of these five main areas there are yet 

 smaller areas in which the movements of localized regions of the body are 

 in part represented and which are indicated in diagram (Fig. 254) by corre- 

 sponding words. The words in the areas marked, eyes and head, face, 

 arm, trunk, and leg, indicate the location of nerve-cells which through the 

 discharge of nerve impulses excite to contraction the muscles which im- 

 part to the regions indicated by these words their characteristic movements. 

 A localized irritative lesion of any one of these areas gives rise to convulsive 

 movements of the muscles of the opposite side of the body, similar in char- 

 acter to those resulting from electric simulation of the corresponding areas 

 of the monkey and ape brains. Destruction of these areas from the growth 

 of tumors, softening, etc., is followed by paralysis of the muscles. Electric 

 stimulation of these areas of the human brain for the purpose of localizing 

 obscure irritative lesions prior to surgical procedures on the brain gives 

 rise to the same convulsive movements. 



Language. The succession of motor acts by which ideas are expressed, 

 is known as language, which may be divided into (i) articulate or spoken, 

 and (2) written. 



The expression of ideas both by words and signs depends primarily 

 on the power of reviving the images or memories of words and letters heard 

 and seen; and secondarily of the power or reviving the images or memories 

 of the muscle movements which were previously employed in an effort to 

 imitate or reproduce the words (speech) or the verbal signs (writing). 



Clinico-pathologic investigations have shown that words or letters heard 

 and seen have areas of representation in the cortex, in the general auditory 

 area, in the supra-marginal convolution and angular gyrus respectively (Fig. 

 252). Destruction of these areas is followed by word-deafness and word- 

 blindness. The same methods of investigation have shown that the muscle 

 movements employed to reproduce the words and the verbal signs also 

 have areas of representation in the cortex; the former in the sub-frontal 

 convolution (Fig. 252), and probably in the adjacent region, the island of 

 Reil, on the left side in the great majority of people; the latter in front of 

 the arm region of the general motor area. Destruction of these areas is 

 followed in the first instance by a loss of the power of executing the move- 

 ments of the muscles employed in speech, and in the second instance, of 

 those employed in writing. 



These different areas are connected with one another by association 

 fibers, and, taken collectively, constitute the language zone. Their situ- 

 ation and relations are shown in Fig. 255. In this figure the dotted lines 

 coming from the ear (a) and the eye (v) represent the auditory and visual 

 tracts through which nerve impulses pass to the auditory (A) and the visual 

 centers (V) respectively. Similar lines coming from the muscles involved 



