718 SPECIAL PHYSIOLOGY. 



lift it from that fluid, cut it in pieces, and drain it upon a proper 

 filter. 



The composition of the clot and serum may be inferred from the 

 facts stated in p. 79. When portions of the clot are examined under 

 the microscope, the solidified fibrin is seen in the form of exceedingly 

 minute fibrillae, not more than the s^i^th or ^oJiFTi^ ^ an ^ nc ^ in 

 diameter, nearly straight, subdividing dichotomously, and sometimes 

 assuming the appearance of rows of minute particles. These fibrilke 

 are most perfect when the blood coagulates slowly. The red corpus- 

 cles in the clot are no longer separate from each other, so as to be 

 freely mobile, as in the circulating blood, but have run together in ad- 

 herent masses or columns, which have been compared to overlapping 

 rows or piles of coins ; the white corpuscles are also entangled, but 

 not in groups, though, under certain circumstances, they collect more 

 abundantly in the upper part of the clot. 



As freshly drawn blood coagulates, it gives off a vapor known as 

 the halitus of the blood. A minute quantity of ammonia escaping in 

 this haiitus, is also evolved. (Richardson.) No carbonic acid escapes, 

 as was once supposed. An odor, often characteristic in the case of 

 different animals, is likewise perceptible, not so much during the co- 

 agulation of the blood, as before that event takes place, when the blood 

 is hottest. During coagulation, no heat is evolved, the temperature of 

 the blood, indeed, being already lowered, more or less, before this phe- 

 nomenon begins. 



The coagulation of the blood is influenced by many circumstances, 

 which determine its rapidity, and modify the characters of the clot 

 itself, as to form, color, and consistence. 



The external conditions which accelerate the formation of the clot, 

 are rest, or, on the contrary, very active stirring, moderate increase 

 of heat, exposure of the blood to air, its slow escape from an artery 

 or vein, its reception into shallow vessels, and contact with rough or 

 multiplied surfaces, or with foreign solid bodies, and, in certain cases, 

 its slight dilution with water. Within the body the circumstances 

 which favor the coagulation of the blood, are certain enfeebled states of 

 the system, frequent bleedings, laceration of the vessels from which the 

 blood escapes, inflammation of the coats of the vessels, or of the lining 

 membrane of the heart, and so-called atheromatous, or other deposits, 

 upon the vessels or upon the valves. 



On the other hand, coagulation is retarded, or interrupted, by move- 

 ment, cold, heat beyond a certain temperature, the exclusion of air, 

 as by covering the blood with a stratum of oil, its rapid escape from a 

 vein or artery, its reception into deep vessels, its contact with smooth 

 surfaces, its exemption from the intrusion of foreign solid bodies, and also 

 by the addition of strong solutions of neutral alkaline salts, or of minute 

 quantities of ammonia. Moreover, it is retarded by the admixture of 

 certain vegetable substances containing narcotic and sedative alkaloids, 

 such as opium, hyoscyamus, belladonna, aconite, and digitalis, and even 

 by strong infusions of tea and coffee. In the case of the addition of 

 strong solutions of neutral salts, and of many other substances, subse- 

 quent dilution of the mixed blood, by adding water to it, is followed 



