INTRODUCTION. 



13 



The blood is the fluid into which the food-stuffs are 

 taken up. It consists of an almost colourless liquid, 

 together with an innumera,ble number of exceedingly- 

 minute blood-corpuscles. 



The blood flows through the body in a system of 

 tubes, or blood-vessels, which branch repeatedly, and 

 at last become merged in the microscopic capillary 

 blood-vessels. These capillaries are present in nearly 

 all parts of the body except the epidermis and epi- 

 dermal structures (hairs, feathers, scales, etc.). They 

 have exceedingly thin walls, which present no re- 

 sistance to the passage of the nutritious substances 

 contained in the blood, so that these can be absorbed 

 by those parts of the 

 body which lie between 

 the individual capillary 

 vessels. The central 

 organ of the circulation 

 is the heart, an enlarged 

 part of the vascular 

 system, possessing thick 

 muscular walls. By con- 

 traction of these, the 

 blood is driven out of 

 the heart (Fig. 6, H); 

 and its exit is possible 

 on one side only (a), as 

 at the other side (&) 

 there is a valve, which 

 closes when the heart 

 contracts. The vessel 

 into which the blood 

 leaving the heart enters is termed an artery (S.A.) 

 It divides into several branches, also known as 

 arteries, and the smallest arteries pass into capillaries, 

 which again are connected with veins, which join 

 larger and larger veins, until finally one or a few 

 open into the heart (A). 



Fig. 6. — Diagram of the Courae of 

 the Circulation. 



