440 - Multicellular Animals, Especially Man 



keep on beating for more than a month after 

 it has been excised from the body. This 

 means that the contractions of the heart do 

 not depend upon excitations from the cen- 

 tral nervous system, although in the intact 

 body, the strength and frequency of the 

 heartbeat are altered considerably under the 

 action of the cardiac nerves. 



Each beat of the heart represents a single 

 twitch, and it is not possible for the heart 

 to undergo a tetanic contraction, because 

 the refractory period (p. 449) of cardiac mus- 

 cle extends into the contraction period. Con- 

 sequently a second stimulus never elicits a 

 contraction unless it comes after the heart 

 has started to relax. 



Sometimes visceral muscle also contracts 

 and expands in rhythmic fashion, even in the 

 absence of any apparent external stimula- 

 tion. But the most important characteristic 

 of visceral muscle is its capacity to remain 

 in a contracted or semicontracted state even 

 while "at rest." A sustained contraction, or 

 tetanus, in skeletal muscle involves a series 

 of excitations coming in usually at the rate 

 of about 50 per second, and skeletal muscle 

 continues to expend extra energy so long as 



the tetanus continues. But in visceral muscle, 

 a state of sustained contraction, which is re- 

 ferred to as tonus, does not involve continu- 

 ous excitation, nor does tonus demand any 

 extra metabolism above that of the resting 

 state. Thus any degree of tonus in the range 

 between complete contraction and complete 

 relaxation can be maintained with perfect ef- 

 ficiency, and extra metabolic activity is de- 

 manded only when the tonus is to be in- 

 creased or decreased. Visceral muscle is not 

 called upon for quick reversible movements, 

 and consequently there is no need for visceral 

 muscle to ready itself instantaneously foi 

 another full-scale contraction. Thus visceral 

 muscle is able to maintain itself at any given 

 status, and visceral muscle expends energy in 

 shortening or lengthening its fibrils only 

 when an appropriate stimulus is received. 



GLANDS 



These specialized effectors of secretion 

 have been discussed previously (Chaps. 1 1 

 and 16), but a few general points will be 

 considered in the present connection. 



The exocrine glands secrete their products 



DUCT 

 GLAND CELL 



L VENULE 

 CAPILLARY 

 ARTERIOLE 



Fig. 24-9. Gland cells in relation to their blood 

 supply. Each gland cell extracts substances from 

 the blood (and lymph) and delivers them (with or 

 without chemical alteration) into the duct system 

 of the gland. 



