VERTEBRATA. 243 



the functions necessary for animal or vegetable life belong 

 equally to every atom of the mass. In Chap. V, Sec. 7, 

 it was shown that the simplest plants and animals differ 

 from the highest, or more complex, only in the modi- 

 fication of some parts of the structure to serve special 

 functions. Thus locomotion is served by the change of 

 bioplasm into muscle, or bone, external protection by 

 transformed epidermal bioplasm, as described in Chap. 

 IV. To regulate and harmonize the complex organs of 

 digestion, respiration, circulation, and secretion, and to 

 conduct sensation and motor force, seems to have been 

 the object of the change of bioplasm into nervous matter. 



Nerve matter exists in the form of cells and of fibers. 

 The cells are soft and grayish, and are generally found 

 accumulated in masses or ganglia, sometimes called 

 nerve-centers. The fibers are of two kinds, one soft and 

 nucleated, the ganglionic or sympathetic fibers, and or- 

 dinary nerve-fibers. 



These latter are for a great part of their length in- 

 closed in a transparent sheath, which coagulates after 

 death into a white substance the white substance of 

 Schwann. A number of these fibers, thus ensheathed, 

 are bound in bundles, which are called nerves. Some of 

 these fibers proceed or conduct impressions from the 

 surface, or from the different organs where they are 

 found, toward the gray centers only, and are called af- 

 ferent or sensory nerves. Others conduct an influence 

 from the centers to contract or move the muscles, and 

 are called efferent, or motor nerves. Thus, on receiving 

 any impression, as the prick of a pin, an afferent nerve 

 conducts the impression to the center, from which an 



