256 MANUAL OF PHYSIOLOGY. 



the arteries are constantly over-full, their elastic walls being dis- 

 tended by the pumping of the heart, which fills the aorta and 

 arteries more quickly than they can empty themselves, until the 

 adequate pressure is attained through the contracting arterioles. 

 The arterioles are the chief agents in resisting the outflow, and 

 keeping up the arterial pressure. 



THE HEART. 



The heart of man and other warm-blooded animals may be 

 said to be made up of two muscular sacs, the pulmonary and 

 systemic hearts, or, as they are commonly termed, the right and 

 left sides of the heart, between which no communication exists in 

 the adult. Each of these sacs may be divided into two portions: 

 the one, a kind of antechamber, which receives the blood from 

 the veins, is called the auricle, and has very thin walls ; the other, 

 the ventricle, is the powerful muscular chamber which pumps the 

 blood into the aorta and distends the arteries. (See Figs. 113 

 and 114.) 



In the empty heart the great mass of the organ, which forms 

 a blunted cone, is made up of the ventricles, while the flaccid 

 auricles are found retracted to an insignificant size at its base. 

 The four cavities have about the same capacity, namely, about 

 six ounces or eight cubic inches when distended. 



The walls of both the auricles are about the same thickness, 

 while the amount of muscle in the wall of the ventricle differs 

 materially on the two sides. The wall of the left ventricle, in- 

 cluding that part which forms the iuterventricular septem, is 

 nearly three times as thick as that of the right or pulmonary 

 ventricle. 



ARRANGEMENT OF MUSCLE FIBRES. 



At the attachment of each auricle to its corresponding ventricle 

 there is situated a dense ring of tough connective tissue, which 

 surrounds the openings leading from the auricles to the ventricles. 

 Similar tendinous rings (zona tendinosa) exist around the orifice 

 of the aorta and pulmonary arteries. These tendinous rings form 



