794 PHYSIOLOGICAL TOPOGRAPHY OF SURFACE OF CEREBRUM. 



impressions. Therefore, the larger portion of the ascending fibers in the posterior 

 columns of the medulla must pass to the central convolutions. By others the 

 superior parietal lobule (P t ) is considered as the seat for impressions of position 

 and movement. The conducting tracts are believed to be situated immediately 

 behind the motor tracts in the internal capsule. It is a noteworthy fact that, 

 in man, on the one hand exclusive loss of muscle-sense or of conception of position 

 has been observed, and on the other hand also pure motor paralysis without a 

 lesion of the former. 



The psychomotor centers may also, at times, be stimulated to activity through 

 psychic influences (grimace, pantomime, gesture), at times be inhibited through 

 psychic shock ("paralyzed by fright," " spell-bound with fear," "speechless from 

 grief," etc.). On stimulation of voluntary movements within certain muscles 

 an inhibitory mechanism in the cortex at the same time becomes effective and 

 renders the adjacent cortical centers inactive. If this inhibition is enfeebled, un- 

 intentional associated movements take place. Thus, in children, for instance, 

 associated movements of the mouth are observed during writing exercises. 



Pathological. Irritation of the psychomotor areas from internal pathological 

 causes may give rise to maniacal motor activity, for example in the state of so- 

 called "possession." Involuntary twitchings in individual muscles due to irri- 

 tation of the motor centers occur in the condition of paramyoclonus multiplex. 

 Deficient activity of the previously referred to inhibition of the psychomotor 

 centers is capable of causing cerebral chorea. In analogy with the ataxic motor 

 states in animals first described by Landois there occurs also in human beings 

 a condition of cerebral ataxia. The cerebral paralysis of childhood is due to 

 degenerative inflammatory processes in the motor areas. In acephalous fetuses 

 marked deficiency in the development of the pyramidal tracts has been observed. 



In man the entire system of the pyramidal tracts may undergo degeneration 

 also from internal causes. Paralysis, spastic contractures, and atrophy of the 

 muscles of the body (on one side or upon both sides) are characteristic, as observed 

 in the amyotrophic lateral sclerosis of the spinal cord of Charcot. In childhood 

 the normal development of the pyramidal-tract system may fail to take place 

 and cerebrospinal paralysis thus result. 



Well-observed clinical cases aid in the localization of the individual motor 

 subcenters. (i) The center for the movement of the leg is situated in the 

 vicinity of the upper extremity of the fissure of Rolando (Fig. 260, C), and in the 

 paracentral lobule (Fig. 262, A B}. (2) The center for the upper extremity is 

 situated in the middle third of the anterior central convolution or somewhat 

 lower (Fig. 260). The center for the thumbs and the fingers is situated in the 

 posterior central convolution below the center for the upper extremity. (3) The 

 center for the facial nerve is situated at the lower extremity of the anterior central 

 convolution (center for the mouth and the lower portion of the face) . The lower 

 third of the anterior central convolution on the left and the adjacent foot of 

 the second and third frontal convolutions contains, on each side, the center for 

 the trigeminus (movement of mastication). The anterior portion of the ante- 

 rior central convolution is connected with the hypoglossal nerve. The most 

 anterior and inferior portion of the anterior central gyrus appears to be the seat 

 controlling the action of the tensors of the vocal bands. The island of Reil con- 

 trols the movements of the vocal bands. (4) The portion of the frontal lobe 

 lying in front of the middle third of the anterior central convolution controls 

 the muscles of the nape of the neck. The centers for the muscles of the trunk 

 are situated upon the surface of the anterior central convolution above the centers 

 for the upper extremity. (5) The external ocular muscles appear to have their 

 cortical center in the angular gyrus (Fig. 260, P 2 l ). The centers for the lateral 

 movement of the head and the eyes are situated in the posterior portion of the 

 second frontal convolution. 



The motor centers may be paralyzed either individually or collectively, and 

 accordingly cortical oculomotor monoplegia, crural (rare), brachial, brachiocrural, 

 linguofacial, and finally faciobrachial forms of monoplegia have been distinguished. 



If the motor centers are irritated by morbid processes particularly 

 hyperemia and inflammation of syphilitic origin, rarely tubercle, tumors, 

 cysts, cicatrices, splinters of bone convulsive movements take place in 

 the related groups of muscles. Those muscles that are usually moved 

 upon both sides appear thus to be stimulated from one center. 



