MOTOR AREAS OF THE CEREBRUM 891 



The knowledge of the segmental innervation of the limb muscles, as 

 furnished in the above table, is of value in the localization of spinal 

 lesions. Paralysis of the extension movements of the wrist and fingers, 

 along with the triceps, for example, usually indicates a lesion of the 

 seventh cervical. It is more particularly in the trunk, however, that 

 the segmental innervation of the muscles is evident. The innervation 

 of the intercostal muscles being unisegmental, one may diagnose the 

 level of a lesion of the upper thoracic region of the cord by observing their 

 behavior during deep inspiration. If the fingers are placed in the in- 

 tercostal spaces, the paralyzed muscles will feel limp and the fingers 

 sink into the space during the act. 



Localization may also be shown by studying the paralyses of the 

 abdominal muscles when the lesion involves one of the lower six thoracic 

 segments. When the patient with a lesion of the eleventh thoracic raises 

 his head from the bed or coughs, the rectus contracts, but the iliac re- 

 gions bulge owing to paralysis of the lower portions of the obliques. Under 

 the same conditions, when the ninth segment is involved the rectus contracts 

 from about one inch above the umbilicus, whereas below this level it remains 

 uncontracted, so that the umbilicus is pulled up. 



The motor fibers leave the ventral horn and pass through the ventral 

 roots of the spinal nerves into the nerve trunks which supply the muscles. 

 In the case of those roots which contribute to the cervical and lumbo- 

 sacral plexus fibers from several segments may be combined into a sin- 

 gle trunk. These trunks moreover break up into branches in which the 

 fibers are sorted out so that motor fibers become separated from sensory 

 fibers to the skin, and so that all fibers from several segments which 

 are passing to neighboring groups of muscles may become combined. 

 Consequently lesions to the nerve trunks of the limbs may show disso- 

 ciation between the paralysis and the loss of cutaneous sensation, and at 

 the same time certain muscles originating from several segments may 

 be affected while other muscles derived trom the same segments remain 

 under normal control. 



Spinal Reflexes 



In addition to voluntary movements, activated by impulses descend- 

 ing through the pyramidal tracts from the brain, many reflex responses 

 of the skeletal muscles may occur which owe their initiation to afferent 

 impulses which do not affect consciousness. These impulses are conducted 

 through the cord by propriospinal neurons, and reach the muscles by 

 traveling over the same peripheral motor neurons which complete the 

 path for voluntary acts. Consequently if paralysis is due to an injury 

 to the motor neurons, either in the ventral horn of the gray matter, or 

 along the peripheral course of their fibers, the muscles can be excited 



