502 TEXT-BOOK OF PHYSIOLOGY. 



Division oj the ventral root fibers is followed by: 



1. Relaxation of skeletal muscles and loss of movement. 



2. Cessation in the discharge of secretions from glands. 



3. Temporary dilatation and loss of the tonus of blood-vessels. 



4. Temporary impairment of the normal activities of the visceral 



muscles from loss of central nerve control; the degree of impair- 

 ment depending on the nature of the viscus involved. 



Peripheral stimulation of the dorsal root fibers produces : 



1. Reflex excitation of spinal centers, in consequence of which there is an 



increased activity of skeletal muscles, blood-vessels, glands, and 

 visceral walls. 



2. Reflex inhibition of spinal nerve centers, in consequence of which 



there may be a decrease in the activities of skeletal muscles, blood- 

 vessels, glands, and viscera. 



3. Sensations of touch, temperature, pressure, and pain. 



4. Sensations of the duration and direction of muscle movements, of the 



resistance offered and of the position of the body or of its individual 

 parts (muscle sensations). 



Division oj the dorsal root fibers is followed by: 



1. Loss of the power of exciting or inhibiting reflexly the activities of 



spinal nerve centers and in consequence a loss of the power of 

 exciting or inhibiting the activities of peripheral organs. 



2. Loss of sensation in all parts to which they are distributed. 



The ventral roots are therefore efferent in function, transmitting nerve 

 impulses from the spinal cord to the peripheral organs which excite them to 

 activity. 



The dorsal roots are afferent in function, transmitting nerve impulses 

 from the general periphery to (a) the spinal cord where they excite its con- 

 tained nerve-centers to activity or to a more or less complete cessation of 

 activity (inhibition), and (b) to the cerebrum where they excite its centers 

 to activity with the development of sensations. 



Segmentation of the Spinal Cord. For the elucidation of many 

 problems connected with the physiologic actions of the spinal cord, as well as 

 of the symptoms which follow its pathologic impairment, it will be found 

 helpful to consider the cord as consisting physiologically of a series of segments 

 placed one in advance of the other, the number of segments corresponding to 

 the number of spinal nerves. Each spin alsegment would therefore comprise 

 that portion of the cord to which is attached a pair of spinal nerves. The 

 nerve-cells in each segment are in histologic and physiologic relation with 

 definite areas of the body, embracing muscles, glands, blood-vessels, 

 skin, etc. 



If the exact distribution of the nerves of any segment were known, 

 its function could be readily stated. By virtue of this segmentation it 

 becomes possible for each segment to act independently of or in cooperation 

 with other segments near or remote, with which they are associated by the 

 intrinsic or associative cells and their axons; and by the same cooperative 

 action the spinal cord itself is enabled to act as a unit. 



