76 ZOOLOGY. 



ways ; either 1st, by the simple absorption into the body, of 

 matters requiring no modification : 2nd, by the more complex 

 function of digestion: the first is almost mechanical: tin- 

 second partakes more of a chemical character. 



In like manner nature, in freeing the body of the useless 

 substances and the necessary results of vital actions, em- 

 ploys two methods of expulsion, exhalation and seen ti,,/,. 

 The first of these processes is almost physical, and depends 

 on the permeability of the tissues ; by the second, peculiar 

 substances are selected from the blood, and eliminated, as it 

 were, in order to be expelled the body ; to effect this, certain 

 organs are required, and these are called secreting organs. 

 The process bears to simple exhalation the relation which 

 digestion does to absorption. 



EXHALATION. 



146. The walls of the bloodvessels are permeable to 

 liquids. By this means, water, gases, and the thinner parts 

 of the blood generally, may pass through these walls by 

 exhalation ; but they do not admit of the passage in this 

 way of the globules, or denser parts of the blood. This is 



E roved by injecting prussiate of potass into the veins of a 

 ^ving dog ; the salt injected may be soon after detected in 

 the thorax and abdomen, mingled with the fluids, which are 

 constantly exhaled on the surface of serous membranes. 

 Spirituous and other liquors are perceivable in the breath >.><m 

 after being taken. 



147. Mechanism of Exhalation. Exhalation is purely 

 a physical phenomenon, however it may be modified by tin- 

 presence of life. Thus, if into the vascular system of an animal, 

 recently dead, an injection be thrown, composed of gelatine, 

 mingled with vermilion finely powdered, the tluid part of the 

 injection will escape into the adjoining tissues, leaving the 

 vermilion particles in the vessels; thus imitating wh;? 

 to take place during life. 



118. In fact, inhalation, already explained, and exha- 

 lation are strictly analogous, ;md take place in the same 

 way : and their activity depends on the spongy and vascular 

 character of the tissue. In another sense, these functions 

 are in the inverse ratio of each other. \\\ pressure on the 

 veins, the exhalation into depending parts may be much 

 increased. 



