836 THE CENTRAL NERVOUS SYSTEM 



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 unsegmental, 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. 



Besides muscular movement, stimulation of the anterior roots in lightly 

 anesthetized animals sometimes causes evidence of general reflex re- 

 sponse and of pain. The explanation is that there are present in the an- 

 terior root certain sensory fibers which are derived from the posterior root 

 but recur in the anterior, so as to reach the membranes of the spinal 

 cord where they terminate. The stimulation of the peripheral end of the 

 motor root must produce, therefore, the same reflex responses as stimu- 

 lation of the central end of the sensory root. Stimulation of the cen- 

 tral end of a motor root has of course no effect. 



THE POSTERIOR ROOT 



The posterior root is the pathway by which impulses of the various 

 receptors enter the spinal cord. Section of any considerable number of 

 posterior roots causes therefore, anesthesia of the corresponding skin 

 and muscle areas, but such a result does not become evident when one 

 root alone is cut, because the sensory area supplied by each root over- 

 laps at least half of that supplied by the neighboring roots. Although 

 it is often difficult to distinguish the segmental distribution in the 

 ramification of the fibers of the motor roots by finding what muscles 

 they influence, this is more evident in the case of the sensory roots. On 

 the trunk itself this segmental arrangement is very plain, but in the 



