136 INTRODUCTION TO GENERAL PHYSIOLOGY 



number of bones. Apparently the iron from the old corpuscles is 

 utilised in the process, being conveyed from the liver to the marrow 

 in some way. It might seem a somewhat remarkable situation for 

 this important process, but the reason seems to be that the channels 

 for the blood must be sufficiently thin-walled and delicate to 

 permit the newly-formed corpuscles to pass into them, and the pro- 

 tection afforded by the solid bones is advantageous. 



The plasma contains two or three different kinds of proteins in 

 colloidal solution, together with various salts, of which the chief 

 is sodium chloride, organic foodstuffs, such as glucose and amino- 

 acids, and waste products, such as urea, in small amount. It also 

 contains fat in very finely emulsified form. The proteins do not 

 serve as food materials. Certain of them take part in that kind of 

 "setting" of blood into a jelly, which is called "clotting," and will 

 be referred to again presently. Other functions of the proteins 

 will be better understood later. The salts are necessary in con- 

 nection with the maintenance of the correct properties of the cell 

 membranes. Without them the functions of the living cells come 

 to an end. 



Internal Secretions. We have seen that food materials, includ- 

 ing oxygen, are supplied to all the tissues through the medium of 

 the circulating blood, and that the carbon dioxide produced by 

 oxidation is removed. Further, the various chemical waste pro- 

 ducts are carried away to be got rid of through the kidneys. But 

 we have also seen that amongst these chemical products there are 

 certain substances, made by glandular secreting organs, which 

 substances pass directly into the blood and have powerful physio- 

 logical effects on various organs and tissues. In many cases these 

 substances are essential to life, so that disease or removal of the 

 organ producing one of them results in death from various morbid 

 conditions. Such are the thyroid gland in the front of the neck, 

 the suprarenal glands above the kidneys, and others. The absence 

 of the normal thyroid gland leads to a swollen state of the tissue 

 under the skin, to mental deficiency and other abnormal symptoms. 

 The absence of the suprarenals results in what is called " Acldison's 

 disease." The absence of certain cells in the pancreas leads to 

 diabetes, and so on. In other cases, as in that of the " interstitial " 

 cells of the sexual glands, ovary and testis, profound changes 

 depend on their internal secretions. Details of the numerous 

 organs of this kind will be found in the larger text-books, but we 

 see that there exists an extensive series of substances which act as 

 chemical means of co-ordination between different parts of the body, 

 and have been called "chemical messengers" or "hormones." It 

 seems not unlikely that every kind of tissue produces some sub- 

 stance of this kind, but it is clear that we cannot remove the whole 



