164 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



there is a reserve of an organic compound (phosphocreatine) ready to break 

 down explosively and liberate energy at the instant the nervous impulses affect 

 the muscle. 



These chemical changes are a part of the intricate workings of muscle. They 

 and others are going on in every animal motion that we see, the quick whirring 

 of the hummingbird's wings, or the movements of the bagpiper who at the 

 same time marches, blows into the bag, and fingers the keys for a Highland 

 fling. 



Involuntary Muscle 



Smooth muscles contract and relax slowly, skeletal ones rapidly; these 

 processes take several seconds in the former, less than one second in the latter. 

 Smooth muscles may hold a certain degree of contraction for a long time with- 

 out apparent fatigue and with great economy of energy. Smooth muscle cells 

 are spindle-shaped, each with a single nucleus and minute contractile fibrils 

 running lengthwise in the cell. None of them is cross-striped, hence the name 

 smooth muscle. 



They are never attached to bone and rarely have tendons (Fig. 10.8). In 



Fig. 10.8. Integumental or skin muscles 

 of a horse, by means of which the skin may 

 be "shuddered" and flies dislodged especially 

 on the neck and shoulders. Such muscles are 

 practically absent on the flanks. (Redrawn 

 from Walter and Sayles: Biology of the 

 Vertebrates, ed. 3. New York, The Macmil- 

 lanCo., 1949.) 



the vertebrate body they occur mainly in the hollow organs of the body cavity, 

 the stomach, intestines, the urinary bladder, the uterus, also in the blood 

 vessels and the air passages of the lungs. In arteries the individual cells are 

 curved in circular layers around the tube; in the intestine they form circular 

 and also longitudinal layers. By the contraction and relaxation of circular 

 layers the intestine executes its peristaltic waves of contraction and relaxation, 

 bulges out in some places, squeezes in at others, shortens and lengthens much 

 as an earthworm does with the rhythmic deliberations characteristic of smooth 

 muscle. 



In their control of skeletal muscle, nerve cells act through the long exten- 

 sions of the cell body; in smooth muscle whole autonomic nerve cells may be 

 present among the fibers. In addition to their stimulation by nerves, muscle 

 cells are also stimulated directly by movements of one another as waves of 

 contraction pass over them. 



Smooth muscles are never bulky and conspicuous but their functions are 



