48 ELEMENTS OF PHYSIOLOGY 



When eating pork, did you ever see the spinal cord of a hog ? 

 How large was the cord ? Have you seen a hog's brain 

 or the brain of an ox? However complicated nerve tissue 

 may seem to be, it is found to consist of nerve cells and 

 their branches, called nerve fibers (Fig. 48). Some cells 

 are arranged in a distinct mass called a ganglion. 



78. A nerve consists of a great number of cell-branches 

 or nerve fibers, just as a number of telephone wires are 

 sometimes bound together in a cable. Nerve cells grow, 

 become active and die, like other cells, and like other cells 

 they consist of protoplasm with a nucleus and nucleolus. 

 A number of processes branch off from them, some cells 

 giving off only one or two, others many (Fig. 47). One of 

 these processes forms a nerve fiber. The axis, or central 

 part of the fiber, is a continuation of the jellylike proto- 

 plasm of the cell ; this core is the essential part of the 

 fiber. The axis is surrounded in most fibers by a sheath 

 of fatty material (Fig. 48). This is for nourishment 

 and protection of the axis, and it is this which gives to the 

 fiber its characteristic ivory-white appearance. The whole 

 is strengthened by being inclosed in a thin, delicate sheath 

 of connective tissue. Some of the nerves go to the mus- 

 cles, and passing between the bundles of fibers, soon divide 

 into branches, for we have seen that the nerves are bun- 

 dles of separate fibers. They subdivide in the muscles till 

 they ultimately send a single nerve fiber to each individual 

 muscle fiber. 



79. How Nerves and Muscles work together. Suppose 

 you put your hand on a hot stovepipe or poker; it is 

 immediately jerked away. How does this wonderful thing 

 happen ? It is found that the heat of the iron causes a 

 disturbance in a nerve fiber ending just under the skin of 

 the finger. This disturbance travels rapidly along the axis, 

 or core, of the nerve, and is called an impulse. It is not 

 a visible change, but some influence that travels from par- 



