184 



Basic Structure of Vertebrates 



turns outward and passes into either the dorsal or the ventral somatic 

 ramus of the nerve (Fig. 158). These recurrent fibers distribute into 

 all the larger branches of the two rami and thus arrive at their effectors, 

 which are smooth muscles and secretory cells, wherever situated in the 

 body-wall. They even reach the skin, innervating the sweat-glands 

 (Fig. 160) and the muscles attached to hair-follicles. The recurrent 

 fibers are nonmedullated. Therefore a ramus communicans consists of 

 two divisions: the "white" ramus includes the medullated connectors 

 which convey impulses into the ganglion; the nonmedullated recurrent 

 fibers collectively constitute a "gray" ramus, whose fibers conduct im- 

 pulses away from the ganglion. Both sets of fibers are efferent, but the 

 "white" connectors are preganglionic and the "gray" exciters 

 are postganglionic, although bound up in the same nerve-trunk. The 

 spinal nerve by which a recurrent fiber emerges from the autonomic 

 cord is usually not the one by which its connector enters. 



Each cervical spinal nerve is connected by a ramus communicans 

 to a cervical lateral ganglion, two or more to each of the three 

 ganglions — a fact indicating fusion of ganglions. But these cervical 

 rami contain no connector-fibers. They consist entirely of outbound 



SWEAT GLAND 



ASSOCIATION 

 NEURONE 



FERENT NERVE 



'CAPILLARIES 



Fig. 160. Diagram illustrating the nervous mechanism of temperature regu- 

 lation in man. The quantity of secretion of the sweat-glands (and consequently the 

 amount of sweat which may evaporate to cool the body) depends upon the quantity 

 of blood in the capillaries associated with the glands and dermal papillae. Through 

 a reflex arc the circulation is regulated by the temperature of the skin. (After 

 Hough and Sedgwick. Courtesy, Neal and Rand: "Cbordate Anatomy," Phila- 

 delphia, The Blakiston Company.) 



