THE MOTOR CORTICAL CENTERS OF THE CEREBRUM. 781 



If the cerebral cortex is removed from an animal, the irritability of the fibers 

 of the corona radiata disappears completely at about the fourth day, exactly as 

 does that of a peripheral nerve separated from its center. 



As the fibers (of the corona radiata or projection-system of the first order) 

 pass from the cerebral cortex toward the center of the hemispheres, it can be 

 understood that after removal of the cortex, inasmuch as the course of the nerve- 

 fibers in the depth of the hemispheres is followed, the same motor effect can be 

 obtained by irritation of those fibers. If the stimulation be thus continued pro- 

 gressively to the internal capsule, where the conducting fibers lie close together, 

 general contractions of the contralateral muscles will be observed. The motor 

 fibers are irritable also within the cms cerebri. 



Time-relations of the Irritation. According to Franck and Pitres 0.045 second 

 elapses between the moment of stimulation of the cerebral cortex and the move- 

 ment, after subtraction of the muscular latent period and the time for con- 

 duction through the spinal cord and the nerve of the extremity. Bubnoff and 

 Heidenhain found that in morphin-narcosis of moderate degree the contraction 

 became greater and the reaction-time shorter with increasing strength of the 

 stimulating current. After removal of the cortex the total delay in the onset 

 of contraction, after beginning stimulation of the white medullary tissue, is dimin- 

 ished from one-quarter to one-third. The form of the muscular contraction (con- 

 traction-curve) is longer and more extended if the cortex is stimulated than if the 

 subcortical conducting path is stimulated. If the animal (dog) is in a state of 

 marked reflex irritability, these differences do not appear. In either event the 

 contraction takes place rapidly. In case of strong irritation, the muscles of the 

 same side also contract, though somewhat later than those of the opposite side. 

 If the motor point for the foreleg and that for the hind leg are stimulated at the 

 same time, the latter contracts the later. If the stimulus is applied to a motor 

 point forty times in a second the muscles in question make forty individual con- 

 tractions. With forty-six separate stimuli in a second persistent contraction re- 

 sults. In the same animal the same number of stimuli are necessary for the 

 production of sustained contraction, whether the cortical center or the motor 

 nerve or even the muscle is irritated. 



In the case of exceedingly feeble stimulation the phenomenon of summation 

 of stimuli is observed, the muscular contractions commencing only after several 

 at first ineffective stimuli. The time required for the voluntary inhibition of a 

 movement already present is about equal to the time for the voluntarily induced 

 movement. 



The situation of the motor centers in the brain of the dog can be seen in Fig. 

 258, I and II. For purposes of orientation it should be stated that the surface 

 of the brain in the dog exhibits two primary fissures, the cruciate sulcus (S) which 

 intersects the longitudinal sulcus, dividing the hemisphere in its anterior third, 

 almost at a right angle. The second primary fissure is the fossa of Sylvius (F). 

 Four primordial convolutions are arranged in a definite relation to these primary 

 fissures. The first primitive convolution (I) surrounds with marked flexion the 

 sharply defined fossa of Sylvius (F). The second primitive convolution (II) 

 passes almost parallel to the first. The fourth primitive convolution is bounded 

 in the middle line by the convolution of the opposite side. It surrounds the 

 cruciate sulcus (S) anteriorly, so that the portion lying in front of this can be 

 readily differentiated as the precruciate gyrus from the postcruciate gyrus lying 

 behind it. The third primitive convolution (III) is in general parallel with the 

 fourth. 



In Fig. 258, I and II, the situations of the motor centers are indicated by 

 dots, although their position varies somewhat and may even be different upon the 

 two sides of the brain. It should, however, be stated that the individual centers 

 do not have merely a punctate extent, but that in accordance with the size of 

 the animal they represent areas the size of a pea and larger, whose central points 

 are indicated by the dots in the illustration. 



Fritsch and Hitzig isolated the following motor centers: (i) For the mus- 

 cles of the nape of the neck: a second center was found by Werner below 7. 

 (2) For the extensors and abductors of the foreleg. (3) For flexion and rotation 

 of the foreleg. (4) For the movements of the hind leg, which Luciana and Tam- 

 burini were able to separate into two centers with antagonistic effects. (5) For 

 the muscles of the face, or the center for the facial nerve (according to these 

 investigators often more than 0.5 cm. in diameter). Ferrier has discovered the 

 following additional centers: (6) 'For the lateral wagging movements of the tail. 



