NERVOUS TISSUE 135 



frame; those of sensation; those of voluntary motion; .... these nerves are some- 

 times separate, sometimes bound together; but they do not, in any case, interfere with 

 or partake of each other's influence." This brilliant discovery was verified by physio- 

 logical experiments to determine "whether the phenomena exhibited on injuring 

 the separate roots of the spinal nerves corresponded with what was suggested by their 

 anatomy." Bell found that such was the fact. (An Exposition of the Natural 

 System of the Nerves of the Human Body, with a republication of papers delivered 

 to the Royal Society, London, 1824.) 



It was at first supposed that the nerves grew out from the cord and brain and 

 acquired connections with their end-organs; but the apparent difficulty which the 

 fibers would have in reaching them, and the fact that the connections must be es- 

 tablished before the nervous system can be functional, have led to the idea that the 

 nervous and muscular systems are connected at all stages of their development. In 

 tadpoles, however, Harrison has shown that such connection is not an indispensable 

 requisite for the normal development of the muscles, since they are formed in a normal 

 manner after the medullary tube and neural crest have been removed from the entire 

 posterior portion of the body. He finds further that nerves grow out into the adjacent 



A B 



FIG. 127. THE GROWTH OF NERVES IN TISSUE CULTURES. (Harrison.) 



A, Two views of the same nerve fiber taken twenty-five minutes apart, during which time the fiber has 

 grown 2On', B, Two views of another fiber, at lower magnification, taken fifty minutes apart. 



tissues from transplanted portions of the medullary tube. Therefore he concludes 

 that the nerves normally grow out to their end-organs and unite with them, but that 

 this takes place very early in development, when the paths are quite direct. Subse- 

 quent growth of the body causes the muscles to shift about and become widely sepa- 

 rated from the central nervous system, so that the nerves become greatly elongated 

 and follow irregular courses (Amer. Journ. Anat., 1904, vol. 3, pp. 197-220; 1906, 

 vol. 5, pp. 121-131). 



The participation of the mesoderm in the formation of nerve fibers has repeatedly 

 been asserted, and some authorities now consider that the long fibers passing from the 

 spinal cord to distant muscles are formed from chains of cells, either mesodermal 

 or ectodermal. Certain of Harrison's experiments were designed to show whether 

 the nerve fibers are formed by peripheral cells or grow out from the central nervous 

 system. In tissue, cultures, made by placing fragments of the medullary tube of tad- 

 poles in lymph, at a stage when the tube consists entirely of round cells, he observed 

 the actual growth of the fibers. Examined after a day or two of cultivation, in a 

 considerable number of cases, they were seen extending out into the lymph clot (Fig. 

 127). Harrison concludes that the nerve fibers begin as an outflow of hyaline proto- 



