260 THE BLOOD. 



ThQ fatty matters exist in the blood mostly in a saponified form, ex- 

 cepting soon after the digestion of food rich in fat. At that period, the 

 emulsioned fat finds its way into the blood, and circulates for a time 

 unchanged. Afterward it disappears as free fat, but remains partly in 

 the saponified condition. 



The saline substances of the plasma are principally sodium and potas- 

 sium chlorides, phosphates, and sulphates, together with lime and mag- 

 nesium phosphates. Of these the sodium chloride is the most abundant, 

 constituting nearly 40 per cent, of all the mineral ingredients. The 

 sodium and potassium phosphates are of great importance in providing 

 for the alkalescence of the blood plasma, a property which is essential 

 to the performance of the functions of nutrition and even to the im- 

 mediate continuance of life ; since it is the alkaline condition of the 

 plasma which enables it to absorb from the various tissues the car- 

 bonic acid produced in their substance and return it to the centre of the 

 circulation, for elimination by the lungs. The sodium and potassium 

 carbonates also take part in the production of this alkalescence, and in 

 the herbivorous animals are its principal cause; while in the carnivora 

 the alkaline phosphates alone are to be found in the plasma in appre- 

 ciable quantity. In the human subject, under the use of an ordinary 

 mixed animal and vegetable diet, both the alkaline phosphates and car- 

 bonates are present in varying proportions. 



The earthy phosphates of the plasma, which are by themselves in- 

 soluble in alkaline or neutral fluids, are held in solution in the blood by 

 union with its albuminous ingredients. 



Coagulation of the Blood. 



A few moments after the blood has been withdrawn from the vessels, 

 a remarkable phenomenon presents itself, namely, its coagulation or 

 clotting. This process commences at nearly the same time throughout 

 the whole mass of the blood, which becomes first somewhat diminished 

 in fluidity, so that it will not run over the edge of the vessel, when 

 slightly inclined ; while its surface may be gently depressed with the 

 end of the finger or a glass rod. It then becomes rapidly thicker, and 

 at last solidifies into a uniformly red, opaque, consistent, gelatinous 

 mass, which takes the form of the vessel in which the blood was received. 

 The process usually commences, in man, in about fifteen minutes after 

 the blood has been drawn, and is completed in about twenty minutes. 



The coagulation of the blood is dependent upon the presence of its 

 fibrine. This fact may be demonstrated in various ways. In the first 

 place, if freshly drawn frog's blood be mixed with a solution of sugar, 

 of the strength of one-half per cent., and placed upon a filter, the blood- 

 globules will be retained upon the filter, while a transparent colorless 

 liquid passes through, which after a time coagulates like fresh blood. 

 Secondly, if horse's blood, which coagulates more slowly than that of 

 most other warm-blooded animals, be drawn from the veins into a cylin- 

 drical glass vessel and allowed to remain at rest, by the time coagulation 



