178 CIRCULATION. 



the lungs and sends it to the system. 1 It must be borne in 

 mind, however, that though the two sides of the heart are 

 distinct from each other, their action is simultaneous ; and in 

 studying the motions of this organ, we will find that the 

 blood is sent simultaneously from the right side to the lungs, 1 

 and from the left side to the system. It will not be necessary, 

 therefore, to separate the two circulations in our study of 

 their mechanism ; for the simultaneous action of both sides 

 of the heart enables us to study its functions as a single 

 organ, and the constitution and operations of the two kinds 

 of vessels do riot present any material differences. 



For convenience of study, the circulatory system may be 

 divided into heart and vessels, the latter being of three 

 kinds : the arteries, which carry blood from the heart to the 

 system ; the capillaries, which distribute the blood more or 

 less abundantly in different parts of the system ; and the 

 veins, which return the blood from the system to the heart. 

 The function of each of these divisions may be considered 

 separately. 



Action of the Heart. 



Physiological Anatomy of the Heart. The heart of the 

 human subject is a pear-shaped, muscular organ, situated 

 in the thoracic cavity, with its base about in the median 

 line, and its apex at the fifth intercostal space, midway be- 

 tween the median line and a perpendicular dropped through 

 the left nipple. Its weight is from 8 to 10 ounces in the 

 female, and from 10 to 12 ounces in the male. It has four 

 distinct cavities : a right and a left auricle, and a right and 

 a left ventricle. Of these, the ventricles are the more capa- 

 cious. The heart is held in place, or may be said to be 

 attached, by the great vessels, to the posterior wall of the 

 thorax, while the apex is free and capable of a certain degree 



1 In some animals, as the dugong, the division between the two sides of the 

 heart is very marked. The heart is really double, having two points, the two 

 sides joined together only at the base. 



