Integrative Systems 



171 



ventral root, usually emerging from the cord in several small rootlets, 

 consists entirely of motor fibers derived from cells in the gray matter 

 of the cord (Fig. 151). 



The fibers which enter the cord via the ganglion-cells of the dorsal 

 root come from exteroceptors (in the skin), interoceptors, and proprio- 

 ceptors — i.e., they include both somatic and visceral conductors (Fig. 

 152). In many vertebrates, but ordinarily not in mammals, the dorsal 

 root contains a relatively small number of visceral motor fibers. These 

 come from cells in the gray matter and pass through the spinal gan- 

 glion, but have no connection with the cells in the ganglion. 



Of the fibers that constitute the ventral root, some are somatic and 

 some are visceral (Fig. 152). The somatic fibers innervate the striated 

 (voluntary) muscles of the body-wall. The visceral fibers are connected, 



I CLARKE S COLUMN 



SOMATIC A A^-H 

 MOTOR %le\ 

 GANGLION 

 CELLS 



VISCERAL MOTOR 

 FIBERS 



Fig. 152. A diagram of neurons of the spinal cord and spinal nerves, showing 

 relations of the neurons to one another and to their end-organs. Somatic sensory 

 fibers are shown by continuous lines, somatic motor fibers by fine dots. Visceral 

 sensory fibers are indicated by short broken lines, visceral motor by long broken 

 lines. (After Plate. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, 

 The Blakiston Company.) 



