QO HISTOLOGY. 



The diameter of muscle fibers is greater in large animals than in 

 small ones; it is increased by functional activity; and varies with the 

 general nutrition so that the caliber may become perhaps trebled. It is 

 doubtful, however, if any new striated muscle fibers develop in the adult. 

 Some have said that they are constantly being worn out and that new ones 

 form to take their places, developing from latent myoblasts. It seems 

 to be generally considered that the formation of new fibers ceases in the 

 embryo; muscle destroyed by injury is not restored in the higher animals. 

 The origin of muscle fibers by division of those already formed, rather than 

 by the development from myoblasts, is also generally denied. 



Striated muscle occurs not only in the muscles of the limbs and body 

 wall, but also in the ocular and ear muscles, the diaphragm, the tongue, 

 pharynx, larynx and upper half of the oesophagus, and in parts of the 

 rectum and genital organs. 



NERVE TISSUE. 



Irritability and conductivity have already been mentioned as funda- 

 mental properties of protoplasm. Response to particular irritants be- 

 comes the chief function of certain cells. Thus some cells in the eye are 

 differentiated to react to light; some in the ear respond to sound; the 

 taste cells of the tongue and olfactory cells in the nose are affected by 

 solutions; tactile cells are influenced by pressure, and muscle cells contract 

 at the stimulus of the nervous impulse. The effects of irritation may 

 be conveyed from one part of the cell to another through its power of 

 conduction. Thus when a muscle fiber is stimulated at one point, a wave 

 of contraction may be transmitted along its whole extent; or when an 

 olfactory cell is stimulated, the effects may be conveyed through a long 

 fiber-like basal prolongation toward the brain. For the purpose of 

 connecting these particularly irritable cells there exists a specially modified 

 median longitudinal tract of ectoderm, the nervous system. Some of its 

 cells send out slender prolongations, known as nerve fibers, to meet the taste 

 cells, the auditory cells, the processes of the nasal cells, the cells of the 

 muscle spindles or the epithelial cells of the skin, and to branch in contact 

 with them. The effects of stimulating the various irritable cells enumer- 

 ated, are conducted along these nerve fibers back to the central nervous 

 tract. Such fibers as convey peripheral stimuli to the central system are 

 called afferent or sensory fibers; they are the outgrowths of sensory cells. 

 Another set of nerve fibers grows out from the central tract and branches 

 in contact with muscle cells, smooth or striated. Since they transmit 

 stimuli which cause the muscles to contract they are called motor fibers, and 



