182 GENERAL ZOOLOGY 



ments. If a dog is anesthetized and the ventral roots of the 

 spinal nerves supplying the forelimb are cut, the animal loses 

 control of the muscles supplied by these nerves, though sensitivity 

 to pain in the skin of the limb remains. If the dorsal roots are 

 cut instead of the ventral, sensation in the limb is lost, but 

 ability to move the muscles remains. If both dorsal and ventral 

 roots of the limb nerves are cut, stimulation by an electric current 

 of the distal end of the ventral root causes contractions of 

 muscles in the limb, while stimulation of the central end of the 

 cut ventral root produces no effect. Nor is any effect produced 

 in the limb by stimulating the distal end of the cut dorsal root; 

 but stimulation of the central end of the same root causes 

 symptoms of pain. The dorsal root is therefore sensory in func- 

 tion and the ventral root motor. 



Autonomic Nervous System. — The autonomic nervous system 

 consists of (1) a pair of ganglionated nerve cords, one on each side 

 of the mid-line of the dorsal wall of the body cavity and extending 

 anteriorly into the cervical region; (2) a number of outlying gang- 

 lia, some of which are included within organs; and (3) nerve fibers 

 forming connections between the ganglia and between the ganglia 

 and the central nervous system. In Man the chain ganglia are 

 composed of 3 cervical, 12 thoracic, 4 lumbar, and 4 sacral 

 ganglia, though variations in these numbers are not uncommon. 

 The white rami communicantes consist of both afferent and 

 efferent fibers derived from dorsal and ventral spinal nerve roots, 

 and extending from spinal nerves to the autonomic ganglia; some 

 to the chain ganglia and others directly to more distal ganglia. 

 Of these there is a thoracolumbar group from the first or second 

 thoracic nerves to the second or third lumbar nerves and a 

 sacral group from two or three of the sacral nerves. There is also 

 a cranial group from cranial nerves III, VII, IX, and X. The 

 thoracolumbar group connects with the chain ganglia and with 

 the coeliac, superior, and inferior mesenteric ganglia. The sacral 

 and cranial groups have no connections with the chain ganglia 

 but pass directly to various organs and tissues. Nerve fibers 

 arising in the central nervous system and passing to chain 

 ganglia or to other ganglia of the autonomic nervous system are 

 called preganglionic fibers because their terminations connect 

 with other ganglia, whose fibers (postganglionic) innervate the 

 tissue. Gray rami communicantes connect the chain ganglia 



