SUPPORT AND MOVEMENT 



387 



proud possession. This trend to longer ten- 

 dons and concentration of the muscle's ac- 

 tion farther from the muscle is most beauti- 

 fully illustrated by the leg of the deer. Its 

 lower leg is little more than skin, bones and 

 tendons, yet all the power of the strong leg 

 muscles is transmitted efficiently to the tiny 

 digits that contact the ground. 



Muscle structure 



Muscle is the principal part of the meat 

 that is bought at the market and it usually 

 makes up about 40-50 per cent of the body 

 weight of large animals. Viewed with the 

 naked eye, muscle is seen encased in a 

 sheath of connective tissue which often 

 glistens. The more expensive cuts of meat 

 have bits of fat rippling tlirough the muscle 

 tissue; this simply means that the animal 

 was fat and its obesity extended to its 

 muscles. The color of the muscle may vary 

 with the nature of tlie fibers, and with age; 

 young mammals such as calves have lighter 

 muscle tissue than older beef. Finally, mus- 

 cles of the viscera possess a different tex- 

 ture than those of the skeleton. 



By sectioning the various muscles of the 

 body the real nature of the muscle can be 

 studied. Cuts anywhere through the diges- 

 tive tract will show smooth or involuntary 

 muscle, the structure of which was de- 

 scribed earlier (p. 72, Fig. 4-5). In man, 

 smooth muscles are located in organs of di- 

 gestion and in the skin, as well as in other 

 places. They have to do with those move- 

 ments which are not directly under volun- 

 tary control, such as peristaltic movements 

 of the digestive tract. These muscles are 

 slow to respond to stimuli and the response 

 that eventually occurs is of long duration. 

 For example, certain pains arising in the 

 abdominal region may be caused by the 

 formation of gas in various parts of the in- 

 testines. When the peristaltic wave pro- 

 duces undue stretching of the gut the 

 pain begins slowly, gradually increasing its 

 intensity and finally passing away. This co- 

 incides with the contraction of the smooth 



muscle. If one pricks the intestine of a frog 

 with a sharp needle it may take from 1 to 10 

 seconds before any reaction is noted, but 

 once contraction starts it lasts for a minute 

 or two, clearly demonstrating the charac- 

 teristic of smooth muscle action. 



The microscopic anatomy of skeletal 

 muscle was described earlier (p. 74, Fio-. 

 4-5 ) , so here we need to mention only some 

 of its characteristics. Within each muscle 

 fiber lie numerous fibrils (tiny fibers) sus- 

 pended in the more fluid protoplasm, the 

 sapcoplasm. Differences in the relative 

 amounts of sarcoplasm and fibrils make a 

 difference in the appearance of voluntary 

 muscle tissue. Muscle fibers that contain a 

 great many fibrils and relatively little sarco- 

 plasm are light in color and when the pro- 

 portion is reversed the muscles are dark. In 

 birds such as ducks, where sustained flight 

 for long periods of time is essential, the 

 breast muscle fibers contain more sarco- 

 plasm and are therefore red, whereas the 

 breast muscles of the domestic chicken 

 which flies only short distances, if at all, 

 are white. This is also true of such birds as 

 grouse which fly in short bursts but never 

 for extended periods. It seems that bird 

 muscles designed for sustained activity are 

 red, whereas those that contract for only 

 short periods are white. 



Cardiac muscle, described in an earlier 

 section (p. 74, Fig. 4-5), functions as a 

 unit because of the nature of its cells. As a 

 result of its sustained action, it is dark in 

 color, as one might expect. 



Muscle action 



Even though one is not aware of it, the 

 muscles of the body, both voluntary and 

 involuntary, are under constant mild con- 

 traction. This is essential, for one thing, to 

 keep the blood vessels sufficiently small to 

 maintain adequate blood pressure. This 

 contraction can be observed when a bullet 

 pierces a muscle. The bullet makes a round 

 hole on its way through, but the resulting 

 aperture is a slit, because of the slight 



