226 FUNCTIONS OF NUTRITION. 



remains fluid for a comparatively long time ; if slowly, from a narrow 

 orifice, it coagulates quickly. The shape and structure of the vessel 

 into which it is received also exert an influence. The greater the surface 

 over which the blood comes in contact with the vessel, the more is coag- 

 ulation hastened. If allowed to flow into a tall, narrow, cylindrical 

 vessel, or a shallow plate, it coagulates more rapidly than if received 

 in a hemispherical bowl, in which the extent of surface is less, in pro- 

 portion to its capacity. For the same reason, coagulation takes place 

 sooner in a vessel with roughened surface than in one which is smooth ; 

 and blood coagulates most rapidly when spread out in thin layers, or 

 entangled in cloths or sponges. Hemorrhage, accordingly, continues 

 longer from an incised than from a lacerated wound ; because the blood, 

 in flowing over the ragged edges of lacerated tissues, solidifies upon 

 them, and blocks up the orifice. 



In all cases there is an inverse relation between the rapidity of coag- 

 ulation and the firmness of the clot. When coagulation takes place 

 slowly, the clot becomes small and dense, and the serum is abundant. 

 When rapid, it is followed by imperfect contraction of the coagulum, 

 and incomplete separation of the serum, and the clot remains large, soft, 

 and gelatinous. 



The blood coagulates in the interior of the vessels after stoppage of 

 the circulation. Under these circumstances coagulation takes place 

 less rapidly than in blood withdrawn from the body. In man, as a rule, 

 the blood is found coagulated in the heart and large vessels from twelve 

 to twenty-four hours after death. In most animals, coagulation occurs 

 earlier than this, usually from four to ten hours after death. 



Coagulation of the blood takes place also within the body, during 

 life, from local arrest or impediment of the circulation. Blood ex- 

 travasated into the connective tissue, the substance of an internal 

 organ, or a serous cavity, coagulates after a short time, and forms a clot 

 which takes the shape of the cavity occupied. A ligature, placed upon 

 an artery in the living subject, produces coagulation above the ligatured 

 spot. The clot extends from the ligature backward to the next collateral 

 branch, that is, to the point at which the circulation still continues. In 

 an aneurism the blood in the dilated portion of the artery coagulates on 

 the inner surface of the sac. In these cases, as well within as outside the 

 body, and during life as well as after death, stoppage or retardation of the 

 circulatory movement induces, after a time, the coagulation of the blood. 



It is asserted, however, that simple stoppage of the local circulation 

 during life will not induce coagulation, unless the inner membrane of 

 the blood-vessel be wounded or irritated. According to Burdon San- 

 derson, if blood be imprisoned in the jugular vein of the rabbit by 

 carefully compressing the vessel at two points between transverse nee- 

 dles, so arranged as not to wound or bruise the vascular coats, it will 

 remain fluid in this situation for two days ; while if ordinary ligatures 

 be applied, a coagulum is formed in the isolated portion of the vein. 



From this it would appear that some injury or alteration of the 



