ELEMENTARY PHYSIOLOGICAL CHEMISTRY 189 



between and thereby hold apart the fat globules, are diminished, and 

 consequently the fat globules are dissolved by ether. So long as they 

 are surrounded by caseinogen molecules they are not acted on by 

 ether. Not only alkalies, but also acids can effect this change. 



When the milk stands for some time, the fats, being specifically 

 lighter, rise to the surface to form the cream, and if this be 

 mechanically agitated it solidifies to form butter. Analysis of an 

 ethereal extract of milk shows that the fats are olein 40 per cent,, 

 palmitin 33 per cent., stearin 16 per cent., and about 7 per cent, of 

 lower fatty acids such as butyrin. There are minute traces of lecithin 

 and cholesterin. 



Colostrum. The milk which first appears during lactation is yellower 

 in colour and of higher specific gravity than that secreted later. On 

 boiling, it yields a distinct coagulum of albumin and globulin, and if 

 examined under the microscope it will be found to contain numerous 

 cells colostrum corpuscles in the protoplasm of which fat globules are 

 present. These cells are, in reality, secretory cells of the mammary 

 glands which have been extruded in the first portions of milk. 



CHAPTER VII. 

 BLOOD. 



BY means of the blood the food-stuffs, absorbed from the intestine, and 

 the oxygen, absorbed from the lungs, are carried into the tissues, where 

 a complex chemical process takes place, resulting in the production of 

 tissue-energy, which may be in the shape of muscular movement, heat, 

 glandular activity, etc. In this chemical process the complex food- 

 stuffs are probably first of all decomposed, the resulting substances 

 being then partially oxidised, so that the ultimate effete products 

 pass into the venous blood, partly in a fully oxidised state (C0 2 and 

 H 2 0), partly unoxidised (lactic acid, precursors of urea). 



Structurally, blood consists of minute solid discs called corpuscles^ 

 suspended in a clear fluid called the plasma. 



When the blood leaves the blood-vessels it is a red opaque fluid 

 of peculiar odour and alkaline in reaction. It soon undergoes a change 

 however, in that it solidifies, or clots. If this clot be left standing some 

 time a clear straw-coloured fluid the serum begins to separate out, 

 which gradually increases in amount, so that, after some time, the 

 clot floats free as a compact dark red solid mass,. 



Before proceeding with the chemistry of the blood we must, accord- 

 ingly, study the phenomenon of clotting ; and in order to do this the 



