128 ANIMAL rilYSIOLOGY. 



injury to the animal ; and may be still better studied in 

 some internal parts of small animals operated on for the 

 purpose. Furthermore, by gazing steadily at a bright field, 

 and moving a finger rapidly in front of the eye, some persons 

 are able to bring into view the blood corpuscles coursing in 

 the capillaries of the retina in their own eyes (p. 251). From 

 such data as these, the calculation is made that the blood 

 moves in the capillaries in the human subject at the rate of 

 one or two inches per minute. The movement looks much 

 more rapid when seen under the microscope in a frog's foot; 

 but the reason of that is, that the distance which the cor- 

 puscles travel being magnified, the apparent rate of motion 

 is proportionately increased ; because the rate of movement 

 is the distance travelled in a given time. 



93. The Veins begin by radicles from the capillaries, in 

 like manner as the arteries end in these vessels. The blood 

 moves in them from the capillaries towards the heart, and 

 their course on that account is described from twig to trunk 

 like the course of a river. They are larger than the corre- 

 sponding branches of arteries, and in the limbs they are more 

 numerous. Thus, in the lower limb below the knee, and in 

 the upper limb below the armpit, the main arteries are 

 accompanied with vence comites, that is to say, two or more 

 veins frequently communicating; and there are, besides, 

 large veins beneath the skin without any corresponding 

 artery. The walls of veins are much thinner than arterial 

 walls, owing to their having the middle or muscular and 

 elastic tunic so slightly developed that their principal thick- 

 ness consists of the external or felted coat; and thus it 

 happens that in a wound, an artery cut across forthwith 

 contracts so as to lessen its apparent diameter, while a vein 

 gapes and continues as large as ever. 



The veins give little or no assistance to the flow of blood 

 by elastic recoil, and, indeed, become easily distended to an 

 undue extent; but they present a peculiar provision to prevent 

 the accumulation of pressure within them, and regurgitation 

 backwards, for they are provided here and there with valves. 

 These valves are on the same principle as the arterial valves of 

 the heart, consisting of semilunar pouches; but the pouches 

 look towards the heart, and instead of there being three of 



