488 NUTRITION. 



cles, before the nerves. It is also found that vital properties, apparently lost or destroyed, 

 may be made to return ; as in resuscitation after asphyxia, or in the restoration of mus- 

 cular or nervous irritability by injection of blood. 



The life of a fecundated ovum is the property which enables it to undergo a certain 

 development when placed under favorable conditions; and, by the surrounding condi- 

 tions, its development may be arrested, suspended, or modified. The life of a non-fecun- 

 dated ovum is like that of any ordinary anatomical element. 



The life of an anatomical element or tissue in process of development is the property 

 by virtue of which it arrives at its perfection of organization and performs certain 

 defined functions, as far as its organization will permit. This can also be destroyed, 

 suspended, or modified by surrounding conditions. 



The life of a perfect anatomical element or tissue is the property which enables it to 

 regenerate itself and perform its functions, subject, also, to modifications from surround- 

 ing conditions. 



The life of a perfect animal organism is the sum of the vitalities of its constituent 

 parts; but a being may live with the vitality of certain parts abolished or seriously 

 modified, as a man exists and preserves his identity with a limb amputated. Life may 

 continue for a long time without consciousness, or with organs paralyzed or their func- 

 tion destroyed ; but certain functions, such as respiration and circulation, are indispensa- 

 ble to the nutrition of all parts, and the vitality of the different tissues is speedily lost 

 when these processes are arrested, and the being then ceases to exist. 



These considerations make it evident that it is difficult, if not impossible, to give a 

 single, comprehensive definition of life, a study of the varied phenomena of which con- 

 stitutes the science of physiology. 



The general process of nutrition begins with the introduction of matter from with- 

 out, called food. It is carried on by the appropriation of this matter by the organ- 

 ism. It is attended with the production of excrementitious principles and the develop- 

 ment of certain phenomena that we have not yet studied, the most important of which is 

 the production of heat. We shall have little to say about food, beyond what we have 

 already considered under the head of alimentation, except to classify the alimentary 

 principles with reference to their relations to the general process of nutrition. 



Principles which pass through the Organism. 



All of the inorganic principles taken in with the food pass out of the organism, gen- 

 erally in the form in which they enter, in the fa3ces, urine, and perspiration ; but it must 

 not be inferred from this fact that they are not useful as constituent parts of the body. 

 Some of these principles, such as water and the chlorides, have very important functions 

 of a purely physical nature. It is necessary, for example, that the blood should contain 

 a certain proportion of the chloride of sodium, this substance modifying and regulating 

 the processes of absorption and probably of assimilation. In addition, however, we find 

 the chlorides as constituent parts of every tissue and organ of the body, and they are so 

 closely united with the nitrogenized principles that they cannot be completely separated 

 without incineration. Those inorganic matters, the function of which is so marked in their 

 passage through the body, are found largely as constituents of the fluids and are less 

 abundant in the solids. They are contained in quantity, also, in the liquid excretions ; 

 and any excess over the amount actually required by the system is thrown off in this 

 way. Other inorganic matters are especially important as constituent parts of the tissues, 

 and they are more abundant in the solids than in the fluids. Examples of principles of 

 this class are the salts of lime, particularly the phosphates. These are also in a condition 

 of intimate union with organic matter, and they accompany these principles in all of their 

 so-called vital acts. 



If we except certain simple chemical changes, such as the decomposition of the bicar- 



