136 



HISTOLOGY 



plasm from the nerve cells. The protoplasm is actively amoeboid, and, as a result of 

 this activity, it extends farther and farther from its cells of origin, retaining its pseudo- 

 podia at its distal end. Similarly enlarged "cones of growth," provided with spiny 

 processes, have been observed in preserved tissue by Cajal; and His, from embryo- 

 logical studies, had long maintained that the nerve fibers grow out from neuroblasts 

 in the central nervous system and spinal ganglia. Harrison concludes that his experi- 

 ments "place the outgrowth theory of His upon the firmest possible basis" (Anat. 

 Rec., 1908, vol. 2, pp. 385-410). 



Dorsal and Ventral Rami. Every spinal nerve, near the junction of 

 its ganglionic and motor roots, divides into a dorsal and a ventral branch I 



FIG. 128. THE SYMPATHETIC SYSTEM IN A I6-MM. HUMAN EMBRYO. (After Streeter.) 



nerve; St., stomach. 



or ramus (Fig. 125, E). Each ramus receives both sensory and motor 

 fibers, and is therefore a mixed nerve. The dorsal rami are distributed to 

 the muscles and skin of the back; their terminal cutaneous branches enter 

 the skin along a line extending from the neck down the trunk of the body, 



