THE MOTOR CORTICAL CENTERS OF THE CEREBRUM. 785 



The irritability of the motor centers may be considerably influenced 

 in various ways. Thus, it is impaired by stimulation of sensory nerves, 

 as this lowers the contraction-curve of the muscles and extends and pro- 

 longs the reaction-time. The irritability of the cortical centers appears 

 to be increased only when active reflex muscular contractions occur in 

 connection with severe sensory irritation. It is an especially remarkable 

 fact that in a certain stage of morphin-narcosis a stimulus that is too 

 feeble to induce a contraction becomes at once active if, shortly before its 

 application to the cortical centers, the skin in certain portions of the 

 body is exposed to even slight tactile irritation. The contractions ac- 

 quire a tonic character on strong pressure upon the paw, so that all stimuli 

 that in the normal 'state cause in the centers only transitory excitation 

 now exert a permanent stimulating effect. If, during the tonic con- 

 traction, the skin on the dorsum of the paw is gently stroked, or the face 

 is blown upon, or the nose is gently struck, or the animal is called, or the 

 sciatic is irritated, relaxation of the muscles suddenly takes place. These 

 phenomena are suggestive of the analogous observations on hypnotized 

 individuals. 



It is a further remarkable fact that if contracture of the muscles in 

 question is induced by reflex irritation or strong electrical stimulation of 

 the cortical center, feeble stimulation of the same center, and also of any 

 other cortical region, suppresses the movement. There is thus afforded 

 the peculiar phenomenon that irritation of the same cortical region, in 

 accordance with the intensity of the current employed, excites irritation 

 of the motor apparatus or inhibits an irritation already present. H. E. 

 Hering and Sherrington observed on irritation of the motor centers of the 

 ape, relaxation of the antagonistic muscles, which occurred even when 

 the stimulus for the excitation of the movement in the muscles related 

 to the center was still too weak. With currents of a certain strength 

 they obtained such simultaneous contraction and relaxation not from 

 the same cortical area, but from widely separated areas. Further, in 

 addition to this reciprocal innervation of the true antagonists, there was 

 a complicated relation between various groups of muscles. Thus, for 

 example, on closure of the fist, dorsal flexion occurs at the wrist-joint. 

 The relaxation of the antagonists takes place somewhat in advance of the 

 contraction of the irritated muscle. 



Sherrington stimulated the central stump of the flexor nerves of the 

 leg containing the muscle-sense nerves, and observed at once loss of 

 tone in the extensor muscles stimulated from the cerebrum. 



According to the investigations of Fano and Libertini and others there is in 

 the pref rental region of the dog an inhibitory center for movements, therefore 

 a psycho inhibitory center for the opposite side of the body. Irritation of the 

 contralateral cerebral hemisphere, for example by application of a crystal of 

 sodium chlorid (in the frog), causes inhibition of the irritability of the motor 

 nerves. Also irritation of the contralateral basal portion of the midbrain or a 

 transverse section of the spinal cord may have a similar effect. 



THE SENSORIAL CORTICAL CENTERS. 



The investigations of Ferrier and H. Munk have shown that areas 

 are present in definite portions of the cerebral cortex in which the act 

 of conscious sense-perception takes place. These areas are connected 

 by means of fibers with the nerves of special sense. They are designated 

 also sensorial cortical centers, sense-centers, or, according to the suggestion 



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