THE HUMAN BODY. 



threads are composed of the proteid substance fibrin. When 

 they first form, the whole drop is much like a sponge soaked 

 full of water (represented by the serum) and having solid 

 bodies (the corpuscles) in its cavities. After the fibrin threads 

 have been formed they tend to shorten; hence when blood 

 clots in mass in a vessel, the fibrinous network tends to shrink 

 in every direction just as a network formed of stretched 

 india-rubber bands would, and this shrinkage is greater the 

 longer the clotted blood is kept. At first the threads stick 

 too fkmly to the bottom and sides of the vessel to be pulled 

 away, and thus the first sign of the contraction of the fibrin 

 is seen in the cupping of the surface of the gelatinized blood 

 where the threads have no solid attachment, and there the 

 contracting mass presses out from its meshes the first drops of 

 serum. Finally the contraction of the fibrin overcomes its ad- 

 hesion to the vessel and the clot pulls itself loose on all sides, 

 pressing out more and more serum, in which it ultimately 

 floats. The great majority of the red corpuscles are held back 

 in the meshes of the fibrin, but a good many pale corpuscles, 

 by their amoeboid movements, work their way out and get 

 into the serum. 



Whipped Blood. The essential point in coagulation 

 being the formation of fibrin in the plasma, and blood only 

 forming a certain amount of fibrin, if this be removed as fast 

 as it forms the remaining blood will not clot. The fibrin 

 may be separated by what is known as" whipping" the blood. 

 For this purpose fresh-drawn blood is stirred up vigorously 

 with a bunch of twigs, and to these the sticky fibrin threads 

 as they form, adhere. If the twigs be withdrawn after a few 

 minutes a quantity of stringy material will be found attached 

 to them. This is at first colored red by adhering blood-cor- 

 puscles: but by washing in water they may be removed, and 

 the pure fibrin thus obtained is perfectly white and in the 

 form of highly "elastic threads. It is insoluble in water and 

 in dilute acids, but swells up to a transparent jelly in the 

 latter. The " whipped " or " defibrinated blood " from which 

 the fibrin has been in this way removed, looks just like ordinary 

 blood, but has lost the power of coagulating spontaneously, 



The Buflfy Coat. That the red corpuscles are not an 

 essential part of the clot, but are merely mechanically caught 

 up in it, seems clear from the microscopic observation of 

 the process of coagulation; and from the fact that perfectly 



