46 



ELEMENTS OF PHYSIOLOGY 



cells in the body have lost this power, but the muscle cells 

 retain it. Figure 27 shows a muscle cell of one form. 

 Figure 45 shows muscle cells of another form. There are 

 other forms of muscle cells, but they are all alike in the 

 fact that under certain conditions they get broader and 

 shorter. Suppose we have a row of cells in a muscular 

 tissue. Think, for instance, of the muscle in the upper 

 arm that causes the elbow to bend when the muscle 

 becomes shorter. It is plain that the shortening of the 

 whole muscle will be equal to the sum of the shortening 

 of all the cells in a line extending along 

 the muscle, and its thickening will be 

 equal to the thickening of the individual 

 cells as they lie side by side. But what 

 makes the muscle cells change shape? 

 We have found that the food brought 

 to the muscle by the blood is stored in 

 the muscle cells in a condition just ready 

 for union with the oxygen which has also 

 **** stored in the cells. This union, or 

 combustion, does not occur continually, 

 ^ llt j s ca used when a certain change 

 called a nerve impulse comes to the mus- 

 cle along a nerve fiber; and when the impulse comes, 

 carbonic acid gas, or carbon dioxid, is formed and the 

 muscle cells become broader and shorter. 



77. Nervous Tissue. Knowing, as you do, how micro- 

 scopic the cells are in size, what would you think if you 

 were told that there are cells in the body that have parts 

 which extend several feet in length ! There are cells with 

 branches which reach, for instance, from the backbone to 

 the toes (Fig. 193). A mass of nerve tissue called the 

 brain occupies almost all of the skull, and forms the 

 spinal cord, or spinal marrow. Nerve tissue forms also 

 the glistening white cords, called nerves, going from the 

 brain and spinal cord to all parts of the body (Fig. 46). 



of the Heart. 





