CIRCULATION, RESPIRATION AND EXCRETION 125 



177. Blood plasma. This very complex fluid contains, be- 

 sides about 90 per cent of water, a great variety of substances, 

 the most prominent of which are the proteins, of which two 

 groups are recognized, viz., two or more serum globulins and 

 the so-called serum albumin, which is probably not a single 

 chemical individual. Plasma contains also approximately 

 0.1-0.15 P er cent f dextrose, from o.i to as much as i.o per 

 cent of fat, usually in some soluble form (243), a great variety 

 of so-called extractives which are in part waste products of cell 

 action, and about i per cent of mineral ingredients. 



178. Coagulation. When blood is drawn from the body it 

 usually coagulates or clots within a few minutes. The coagu- 

 lating substance is a globulin called fibrinogen and its coagulation 

 is an enzymatic reac- 

 tion brought about 



by a ferment, throm- 

 bin, believed to be 

 derived from the 

 blood platelets by 

 a very complicated 

 process. The coag- 

 ulated protein con- 

 stitutes the so-called 

 blood fibrin, which 

 entangles within it- 

 self the corpuscles, 

 producing the famil- 

 iar blood clot. While 

 the clot is very bulky 

 the dry blood fibrin 

 amounts to only 0.2- 

 0.3 per cent of the 

 weight of the blood. 



179. The heart. 

 The blood is distrib- 

 uted to all parts of 



FIG. 17. Diagram of mammalian heart. 



a, Left ventricle, b, Right ventricle, c. Left auricle. 

 d, Right auricle. /, Aorta. gg, Pulmonary arteries. 

 - , , , - op.. Pulmonary veins. (Smith, Physiology of the Domestic 



the body by means ol Animals.) 



a most interesting 



organ, the heart, which is in effect a living force pump. 



Figure 17 shows diagrammatically the structure of the mam- 



