562 TEXT-BOOK OF PHYSIOLOGY 



give origin to the autonomic nerve fibers which excite to activity the epithelium 

 of glands, the non-striated muscles in the walls of the blood-vessels and viscera. 



The Rubro-spinal Tract. From its origin and termination this tract 

 may be regarded as a motor tract, though its special function cannot be 

 stated with any degree of accuracy. Associating the red nucleus with the 

 spinal cord segments it has been assumed that it subserves thalamic 

 and cerebellar reflexes. The red nucleus is also associated with the 

 cerebrum and therefore the entire tract may serve as a motor pathway 

 for undetermined movements; possibly for the maintenance of a tonic 

 contraction of muscles. 



The Tecto-spinal Tract and the Vestibulo-spinal Tracts. These tracts, 

 by reason of their origin and association with the cells of the corpora 

 quadrigemina and the cells of the nuclei in the medulla oblongata around 

 which the terminals of the axons of the vestibular nerve arborize, have 

 led to the supposition that they constitute efferent portions of reflex arcs, 

 associated with visual and equilibratory reflexes, coordinating groups of 

 muscles involved in the maintenance of equilibrium both in station and 

 progression. The olivo-spinal tract may be associated in function with the 

 vestibulo-spinal tract. 



The Functions of the Ventral and Dorsal Roots of the Spinal Neives. 

 From the statements in foregoing pages it is apparent that the spinal 

 nerves afford a means of communication between the central nerve system 

 and the remaining structures of the body. The functions subserved by 

 these two groups of nerves may be conveniently summarized in this con- 

 nection as follows: 



The facts presented make it apparent that the anterior or ventral 

 roots contain all the efferent fibers, the posterior or dorsal roots all the afferent 

 fibers. The proofs in support of this view are as follows: 



Stimulation of the ventral root fibers produces : 



1. Tetanic contraction of skeletal muscles. 



2. Discharge of secretions from glands. 



3. Increase in the degree of the contraction, the tonus, of the muscle 



walls of the peripheral arteries. 



4. Variations in the degree of the contraction, the tonus, of the muscle 



walls of certain viscera either in the way of augmentation or in- 

 hibition. 1 

 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, glands, blood-vessels, 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, glands, 

 blood-vessels, and viscera, 



