CIBCULATION OF THE BLOOD. 



51 



this fluid in motion. In other words, bloodvessels and a 

 heart. 



The heart is the centre of the circulatory apparatus : it is 

 a pouch or bag, more or less complex, into which the hlood is 

 returned by the veins, and from which it passes to the body, 

 thus constantly circulating. It is, in fact, a forcing-pump 

 placed at the centre of the circulatory system. 



Most animals, from man to the spider, have a heart ; but 

 the arrangement differs in the various classes of animals. 



The bloodvessels are of two kinds : 1. Arteries, which 

 convey the aerated blood into the various parts of the body. 

 2. Veins, which re-convey the blood to the heart from the 

 body. The arteries, proceeding from the heart, divide and 

 subdivide in their course as they proceed, until they become 

 exceedingly small : the veins follow a different course ; they 

 commence by extremely fine roots, and collect into large 

 branches and trunks as they proceed towards the heart, in 

 the arteries, the motion of the blood is from the heart ; and 

 in the' veins, towards the heart. The arteries terminate, and 

 the veins commence, in a fine network of vessels formed by 

 both; the vessels composing this net- 

 work are called capillary. The heart 

 being thus placed between the termina- 

 tion of the veins and the commencement 

 'of the arteries, the movement of the 

 blood in man and higher animals is a 

 complete circle, the blood always return- 

 ing to the point from which it started ; 

 hence it has been called the circulation 

 of the blood. 



In all animals in which the respira- 

 tion is performed by a special organ, the 

 blood is sent to it from the heart, and 

 retires from it by a special system of 

 canals ; the circulation thus established 

 is called the respiratory or the lesser 

 circulation, whilst that through the 

 body is called the greater. We shall 

 consider, first, the circulation in man, Fig. 38. Capillary Ves- 

 and this will serve as a standard of sels in the Foot of the 

 comparison with others. 



* a, arteries, 

 culating fluids. 



, veins. The arrows point out the direction of the cir- 

 E 2 



