176 CIRCULATORY SYSTEM. 



to be an indispensable constituent, as many animals are entirely 

 deprived of it, and such as naturally are possessed of it, may 

 have its quantity very much reduced by repeated bleedings. The 

 colouring matter is generally an ingredient of the crassamentum, 

 so that the whole of the latter has a red appearance; yet there 

 are some conditions of the body in which a spontaneous separa- 

 tion of it takes place, more or less completely. For example, in 

 inflammatory diseases the bfood does not coagulate so soon as in 

 health; and the red globules, from being naturally heavier than 

 the other constituents of the crassamentum, subside to its bottom 

 and leave it of a white semi-transparent colour. It is this white 

 part upon which depends the whole property of coagulating, and 

 which has been called coagulating lymph. We have, therefore, 

 three constituents of blood manifested by its own spontaneous 

 changes; the serum, the red globules, and the coagulating lymph. 

 Coagulation, contrary to popular opinion, is not assisted by 

 cold, but rather retarded by it: heat assists it.* If the heat be 

 raised to 120, blood will coagulate five minutes sooner than if 

 left at its natural standard, and even sooner than if its tempe- 

 rature be reduced to 50. If blood be frozen quickly, before it 

 has time to coagulate, on being thawed it returns to the fluid 

 state, and will coagulate afterwards. The contact of air does 

 not produce coagulation. The late Dr. Physick, in order to 

 ascertain this point conclusively, took a glass tube, which had 

 a stop cock at each end, and attached one of its ends to the vein 

 of a dog. A current of blood was then conducted through the 

 tube, and while it was flowing, the far stop cock was closed, 

 and immediately afterwards the other; thus, a column of blood 

 was obtained which had not touched the air. After permitting 

 it to remain a proper time, t^ie tube was broken asunder, and 

 the blood found coagulated as usual. Rest is not indispensa- 

 ble to the process, for blood, if shaken in a vial, will still co- 

 agulate. The division of the blood into small masses expe- 

 dites coagulation. Therefore when it flows slowly from the 

 blood vessels, falls from some height, or runs for a distance 

 over the surface of a dish, it coagulates sooner than under op- 

 posite circumstances. The latter are then auxiliary to the blood 

 manifesting the sizy coat, one of the concomitants of inflam- 



* Hunter on the Blood. Hewson. 



