CIRCULATION OF THE BLOOD. 311 



the middle coat is the thinnest so much so, indeed, that, while 

 the arteries when cut remain patulous, the veins collapse. This 

 coat contains both elastic and fibrous tissue : the former gives 

 the vessels some elasticity, while to the latter is attributable the 

 greater strength of the veins as compared with the arteries. The 

 greater thickness of the arterial wall would seem calculated to 

 make these vessels the stronger, but, although possessed of thin 

 walls, still the white fibrous tissue which aids in their formation 

 gives the veins greater resisting power. Valves are to be found 

 in most of the veins, but are absent in those whose diameter is 

 less than 2 mm. ; also from the vena cava, hepatic, portal, renal, 

 uterine, ovarian, cerebral, spinal, pulmonary, and umbilical veins. 

 The valves are so arranged that they permit the blood to flow in 

 the direction of the heart, but prevent its flow in the opposite 

 direction. They consist of a reduplication of the internal coat, 

 together with connective and elastic tissue to give them strength. 

 Like the arteries, they are supplied with vasa vasorum to nourish 

 their walls. 



CIRCULATION OF THE BLOOD. 



The course of the blood, starting from any point, may be 

 traced through the circulatory apparatus. The circulation from 

 the right ventricle through the lungs and back to the left side of 

 the heart is the lesser or pulmonary circulation ; that from the left 

 ventricle through the rest of the body other than the lungs and 

 back to the right side of the heart is the greater or systemic circu- 

 lation. Beginning with the right auricle, the blood flows into this 

 cavity from the venae cavae (inferior and superior) ; thence through 

 the right auriculoventricular orifice into the right ventricle ; thence 

 into and through the pulmonary artery to the lungs ; thence by the 

 pulmonary veins into the left auricle; thence through the left auric- 

 uloventricular orifice into the left ventricle; thence into and 

 through the aorta and the arterial system to the capillaries ; 

 through these vessels to the veins, by which, through the venae 

 cavae, it returns to the right auricle, the place of beginning. 



Cardiac Movements. If the heart is exposed in a living 

 animal a dog, for example it will be seen that the ventricles 

 are at one time in motion and at another time at rest. Each 

 period of motion and rest constitutes a pulsation or a cardiac cycle, 

 and these pulsations recur very rapidly, so much so that the inter- 

 vals are recognized with difficulty. These different states of the 

 heart are better detected by the sense of touch than by that of 

 sight. If the ventricles are grasped by the hand, it will be found 

 that, corresponding with the resting stage, the muscular tissue 

 composing them is soft and flaccid, while during the active stage 

 it is hard and resisting. If these movements are studied still more 

 carefully and analyzed, it will be found that the beginning of the 



