406 ELEMENTARY ANATOMY. [LESS. 



LESSON X. 



THE CIRCULATING SYSTEM. 



1. The CIRCULATING SYSTEM comprises the various chan- 

 nels or vessels by which the nutritive fluid of the body the 

 blood is conveyed to and from every part of the organism. 

 These vessels are of various and very different sizes. 



The largest and most complex part of the circulating 

 system is the heart, which may be considered as the central 

 portion, all the other channels being subsidiary to it. These 

 latter may be divided into three categories : (A), the vessels 

 taking blood from the heart which vessels are called 

 arteries; (B), the vessels taking blood towards the heart 

 which are the veins ; and (C), the minute tubes (capillaries} 

 which convey the blood to the tissues, and intervene between 

 and connect the ends of the arteries and veins- being dis- 

 tinguished from both those kinds of vessels by the absence 

 of muscular fibres in their walls. 



In addition to these vessels, which with the heart constitute 

 what may be called the true circulating system, there are yet 

 other vessels, lymphatics and lacteals, which convey fluid 

 from almost all parts of the body into the veins to which 

 they thus form as it were a supplementary addition. 



The fluids these vessels convey into the veins are (i) the 

 colourless portions of the blood which have exuded from the 

 capillaries, and (2) nutritive juices from the walls of the ali- 

 mentary canal. 



All these parts have been so fully described not only as 

 regards their function, but also as regards their structure in 

 Lesson II. I n of "Elementary Physiology," as to 

 render unnecessary much which must otherwise have been 

 here detailed. 



2. The HEART in man, its form, cavities, apertures, valves, 

 &c., have all been described and figured in the " Elementary 



