56 



ANATOMY. 



arteries with the finest veins. They are quite peculiar in structure. 

 The arteries, branching out as they run from the heart into every 

 vascular tissue of the body, become at last very small, and have very 

 thin coats. Ultimately they end in a fine network of vessels called 

 the capillaries (capillus, a hair), from which the smallest veins com- 

 mence. These small veins at first have very thin coats, but, continu- 

 ally joining together to form larger and larger veins, at length run in 

 a few main trunks to reach the heart again. The heart, arteries, 

 capillaries, and veins form therefore a closed system of chambers and 

 tubes, in which the blood is contained ; and, as we shall see hereafter, 

 whilst the heart and all the bloodvessels are concerned in conveying 

 the blood through the body, it is the delicate capillaries only which 

 permit nutritive material to pass from the blood through their coats 

 into the tissues. 



The ramified course of the bloodvessels generally is well seen in the 

 web of a living frog's foot, Fig. 32 ; the capillary network itself in 

 the same part becomes visible with the aid of a low magnifying power, 

 as in Fig. 33 ; and under a much higher power, Fig. 34, the tubular 

 character and distinct parietes of the capillary vessels, the mode in 



Fig. 34. 



Fig. 33. (After Wagner.) A piece of the frog's web, with a portion of a toe, slightly enlarged, showing 

 the fine capillary network connecting the terminations of the arteries with the commencement of the 

 veins. 



Fig. 34. (Allen Thomson.) Minute piece of the margin of the frog's web, showing the ultimate blood- 

 vessels or capillaries, connecting the end of a small artery with the beginning of a minute vein. The oval 

 blood-corpuscles are seen in these vessels, and the arrows entering and passing out of the artery and vein 

 indicate the course of the blood-current. Magnified about 30 diameters. 



which they connect the fine arteries and veins together, and their con- 

 tained blood, are distinctly perceived. 



In the vascular tissues of the human body also, the capillaries usu- 

 ally form a network, which may consist either of elongated meshes, as 

 in muscle, Fig. 35, a, in tendons, and in nerves ; of a polygonal net- 

 work, as in smooth mucous membranes, b; of long loops, as in the 

 skin, Fig. 36, a; or of close meshes, as in the small intestine, b; or 

 of still closer meshes, as on the ducts of glands, Fig. 37. The finest 

 meshes of capillaries are found in the lungs. The capillary vessels 



