92 Mr. Mayo on Human Physiology. 



to contract. The alternations of action and repose seem 

 natural to it, or to be the result of its structure. It is re- 

 markable that if the heart, yet beating", be placed in warm 

 water, it continues to act more briskly, and for a longer 

 time, than if exposed to the air ; but that if water be in- 

 jected into its blood-vessels, its flesh becomes pale and 

 swollen, and after two or three beats hardens permanently : 

 if its fibres be transversely cut through, their action is 

 stopped at once." 



The causes which combine to give motion to the blood are 

 summed up in the following manner. 



'* During the systole of the ventricles, blood expelled 

 from their cavities and thrown into tlie arteries, urges on 

 that before contained in either arterial trunk, and in its 

 branches, in the capillary vessels, and in the veins, towards 

 the auricle of the opposite side of the heart. During 

 the succeeding diastole many forces continue to operate, 

 which tend to diminish the area of the vascular system, 

 and as the blood is prevented returning into the ven- 

 tricles by the semilunar valves, they serve to propel it 

 towards ihe auricles. These forces consist in the elasticity 

 of the arteries themselves, in the compression of surround- 

 ing elastic organs, in the contraction of muscular parts, in 

 the pressure of the atmosphere. 



" Various causes combine to give effect to these forces in 

 filling the auricles with blood, which operate by taking off 

 or diminishing the atmospheric pressure upon their outer 

 surface. Each auricle during its diastole spontaneously 

 expands : the elasticity of the lungs constantly tends to 

 draw apart the walls of the auricles ; and at the time of 

 each inspiration, while the area of the chest is enlarging, the 

 heart is probably relieved of external atmospheric pressure 

 in the same manner as the lungs, although in a less degree." 



The theory of absorption is a theme of warm discussion 

 up to the present moment. 



The ancients believed, that the veins exercised the function 

 of absorption. In the beginning of the seventeenth century, 

 Asellius saw chyle in a separate and peculiar set of vessels 

 in the mesentery ; these vessels were termed lacteals, and 

 admitted to be the instruments intended for the removal 

 of chyle ; while absorption in other instances continued 

 to be attributed to the veins. In the latter part of the 

 last century? Dr. Hunter made out the universality of 

 the distribution of lymphatic vessels throughout the body, 

 and was led by analogy to suppose that they were the pro- 



