GAINS AN0" LOSSES OF THE BLOOD 173 



much, but probably very little. The greater part of the 

 heat is generated in the tissues, in the muscles and 

 elsewhere, and is given up by the tissues to the blood. 

 So that we may say that the blood gains heat from the 

 tissues. 



These several gains and losses are for the most part 

 going on constantly, but are greater at one time than at 

 another. Thus the gain to the blood from the alimentary 

 canal is much greater some time after a meal than just 

 before the next meal, though unless the meals be very far 

 apart indeed, the whole of the material of one meal has 

 not passed into the blood before the next meal is begun. 

 Again, though the muscles, even when completely at rest, 

 are taking up oxygen and nutritive material, and giving 

 out carbonic acid and other waste products, they give out 

 and take in much more when they are at work. So also 

 certain "secreting glands" as they are called, which we 

 shall study presently, such as the salivary glands, have 

 periods of repose ; it is at certain times only, as when 

 food has been taken, that they pour out any appreci- 

 able quantity of fluid. Hence though they are probably 

 taking up material from the blood and storing it up in 

 their substance even when they appear "at rest, they take 

 up much more and so become much more distinctly means 

 of loss to the blood, when they are actively pouring 

 out their secretions. ^ In the case of the liver the loss to 

 the blood is more constant, since the secretion of bile as 

 we shall see is continually going on, though greater at 

 certain times than at others ; and the materials for the 

 bile have to be provided by the blood. Some of the 

 constituents of the bile, however, pass back from the 



1 The word '■ secretion " is used by physiologists in two senses. Pri- 

 marily it is used to denote the sum total of the processes by which a 

 gland" or organ forms the fluid which it gives out ; thus we say that the 

 salivary glands "secrete" saliva. But the fluid thus formed is itself 

 spoken of as "a secretion." The word "excretion" is usually applied 

 to any fluid which after its formation is useless and requires to be at 

 once got rid of. Thus we say that urine is an excretion which is se- 

 creted {i.e. formed) by the kidneys; and we speak of those secretory 

 structures which get rid of waste as excretory organs. 



