5 THE NERVOUS SYSTEM 



tween the cerebrospinal nervous system, composed of the brain and spinal cord 

 with their associated nerves, and the sympathetic nervous system. But this usage 

 has the disadvantage that it is likely to engender an entirely false notion of the 

 independence of the sympathetic system. 



The spinal nerves take origin from the spinal cord within the vertebral canal 

 and make their exit from this canal through the corresponding intervertebral 

 foramina. As component parts of such a nerve there may be recognized a 

 ventral and a dorsal ramus, a ventral and a dorsal root, and associated with 

 the latter a spinal ganglion. The fibers of the ventral root have their cells of 

 origin within the spinal cord and are distributed through both ventral and 

 dorsal rami. Since they conduct impulses from the spinal cord they are known 

 as efferent or motor fibers. The sensory or afferent fibers of the dorsal roots 

 and spinal nerves arise from cells located in the spinal ganglia. These fibers 

 are also distributed through both ventral and dorsal rami (Fig. 37). 



Metamerism. That the spinal nerves are segmentally arranged, a pair for 

 each metamere, is readily appreciated in the case of the typical body segments 

 of the thoracic region. Here it is obvious that a nerve supplies the correspond- 

 ing dermatome and myotome, or in the adult the skin and musculature of its 

 own segment. While the thoracic nerves retain this primitive arrangement in 

 the adult, the distribution of fibers from the other spinal nerves is complicated 

 by the development of the limb buds and by the shifting of myotomes and 

 dermatomes during the development of the embryo. 



Opposite the attachment of the limb buds the ventral rami of the correspond- 

 ing nerves unite to form flattened plates, and from these plates the brachial and 

 lumbosacral plexuses are developed. Within these plexuses the fibers derived 

 from a number of ventral rami are intermingled in what appears at first to be 

 hopeless confusion. Each nerve which extends from these plexuses into the 

 limbs carries with it fibers from more than one spinal nerve. To determine 

 the exact distribution of the fibers from each segmental nerve has been a very 

 difficult problem, in the elucidation of which the work of clinical neurologists 

 has been of the first importance. A study of the paralyses and areas of anes- 

 thesia, resulting from lesions involving one or more nerve roots within the ver- 

 tebral canal, has contributed much toward its solution. 



Sherrington (1894) attacked the problem of the distribution of the sensory 

 fibers by experimental methods on cats and monkeys. He found that section of 

 a single dorsal root did not cause complete anesthesia anywhere, and attributed 

 this result to an overlapping of the areas of distribution of adjacent spinal nerves. 



