TISSUES. 11 



The functions of Animal life are those of sensation and 

 voluntary motion, as already indicated functions which 

 are generally regarded as belonging exclusively to animajs, 

 and which bring them into relation with the objects about 

 them and with one another, and hence are sometimes 

 called the functions of relation. 



As regards the substance of animals, it may be stated in 

 : general terms, that all parts of their bodies, the hard parts 

 as well as the soft, are made up of tissues, and it may be 

 added here that the same is true of plants. Thus, bones, 



ed in the cavities of their bodies. By absorption, plants secure the material 

 necessary for their nourishment. Animals, on the contrary, especially the 

 higher animals, receive from without only a small portion of their nourish- 

 ment in this way ; but most of their nourishing material is, previous to its 

 absorption, elaborated by the digestive process. If we would prove the 

 fact of absorption in animals, we may place a frog in water, and observe 

 that his weight is soon thereby increased, and this, too, even when it is so 

 arranged that no water enters the mouth. If water be introduced into the 

 stomach of any living animal, and botli orifices of the organ closed, even 

 then the water will disappear, being absorbed by the walls of the stomach, 

 whence it goes to mingle with the blood. But the true Absorbent system, by 

 which the nutritive products of digestion are secured to the blood, is de- 

 scribed on page 43. 



Diycstion, circulation, and respiration, have, for the present, been suffi- 

 ciently noticed on page 9. These acts or processes will be more fully ex- 

 plained farther on. 



Exhalation is the escape of useless substances through the external and 

 internal surfaces of the animal an act depending upon the permeability of 

 the animal tissues. 



Secretion is a function by which certain substances arc selected and elimi- 

 nated from the blood by special organs called glands. Saliva is secreted by 

 the salivary glanils ; bile is secreted by a glandular organ called the liver ; 

 urine is secreted by the kidneys, and so on. 



All the functions and organs by which the animal is freed of the waste 

 products of the body, are often spoken of as the functions and organs of 

 ffrnrtion. The skin, the lungs, and the bladder, are excretory organs, and 

 perform excretory functions. 



Assimilation is the act by which the tissues are built up from the material 

 furnished by the blood, and by which they are sustained in st.renu'th and in 

 growth, and by which, in many cases, they arc restored when destroyed. 



