THE SYMPATHETIC AND PARASYMPATHETIC SYSTEMS 



633 



These fibers retain their medullary sheath and form the so-called ramus 

 albus communicans (W), i.e., a bridge by means of which certain effer- 

 ent cerebrospinal impulses are enabled to enter the sympathetic 

 system (S). The cell-bodies of these neurons form the later omedian 

 group of cells of the anterior horn in the thoracic and lumbar regions 

 of the spinal cord. It is to be noted, therefore, that the anterior 

 root is made up of two groups of efferent fibers, one of which conveys 

 impulses directly to the striated muscles and the other, to the sympa- 

 thetic system. The former are musculomotor (striated muscle) in 

 their function, and the latter, musculomotor (smooth muscle) vaso- 

 motor, secretomotor and pilomotor. 



Fig. 313. — Diagrammatic Representation op the Connection Between the Cerebro- 

 spinal AND Sympathetic Systems. 

 AR and PR, Anterior and posterior roots of the spinal cord; SG, spinal ganglion; 

 N, spinal nerve; W, white ramus; G, gray ramus; S, sympathetic ganglion; I, ordinary 

 motor neuron, the axon of which pursues a straight course to peripheral effector; 

 //, motor neuron, the axon of which enters sympathetic ganglion through the white 

 ramus. Ill, secondary neuron carrying the impulses from II to other parts of sympa- 

 thetic system; IV, secondary neuron; carrying impulses from sympathetic system 

 through the gray ramus to the peripheral effector in the domain of the cerebrospinal 

 system; V, neuron carrying afferent impulses from sympathetic system into cerebro- 

 spinal system by way of spinal ganglion and posterior root. 



Immediately adjoining the ramus albus is another bridge which 

 unites the sympathetic ganglion with a somewhat more peripheral 

 point of the mixed nerve. Its gray color suggests that the fibers 

 composing it are non-medullated and are, therefore, of sympathetic 

 origin. This is the ramus griseus communicans (G) . In some animals, 

 however, the white and gray rami are united into a single trunk and 

 arise from the same segment of the mixed nerve immediately beside 

 the spinal ganglion. It need scarcely be emphasized that the gray 

 ramus forms an afferent connection which enables sympathetic 

 impulses to reach the spinocerebral tracts (IV and V). 



