242 OF THE SECRETIONS. 



retained in the blood with impunity ; or the fluids selected 

 are such as are positively injurious, and cannot remain in the 

 hlood without soon destroying life. These latter are usually 

 termed excretions. 



410. As the weight of the body, except during its period 

 of active growth, remains nearly uniform, it follows that 

 it must daily lose as much as it receives ; in other words, the 

 excretions must equal in amount the food and drink taken, 

 with the exception of the small proportion discharged by the 

 alimentary canal. Some of the most important of these outlets 

 will be now indicated. 



411. We have already seen that all animal tissues admit 

 of being traversed by liquids and gases, This mutual trans- 

 mission of fluids from one side of a membrane to the other is 

 termed endosmose and exosmose, or imbibition and transu- 

 dation, and is a mechanical rather than a vital phenomenon, 

 inasmuch as it takes places in dead as well as in living tissues. 

 The blood-vessels, especially the capillaries, share this property. 

 Hence portions of the circulating fluids escape through the 

 walls of the vessels, and pass off at the surface. This super- 

 ficial loss is termed exhalation. It is most active where the 

 blood-vessels most abound, and accordingly is very copious 

 from the air tubes .of the lungs, and from the skin. The loss 

 in this way is very considerable, and it has been estimated 

 that, under certain circumstances, the body loses, by exhala- 

 tion, five-eighths of the whole weight of the substances re- 

 ceived into it. 



412. The skin, or outer envelope of the body, is other- 

 wise largely concerned in the losses of the body. Its layers are 

 constantly renewed by the tissues beneath, and the outer dead 

 layers are thrown off. This removal is sometimes gradual and 

 continual, as in man; in fishes and many mollusca, it comes off 

 in the form of slime, which is, in fact, a collection of cells de- 

 tached from the surface of the skin ; sometimes the loss is pe- 

 riodical, when it is termed moulting. Thus, mammals cast 

 their hair, and the deer their horns, birds their feathers, serpents 

 their skin, crabs their test, and caterpillars their outer en- 

 velope, with the hairs growing from it. 



413. The skin presents such a variety of structure, in 

 the different groups of the animal kingdom, as to furnish 

 exceUent distinctive characters of species, genera, and even 



