24 HANDBOOK OF PHYSIOLOGY. 



with the chemical composition of the dead body, and it is as well at 

 once to admit that there may be many chemical differences between 

 living and not living tissues; but as it is impossible to ascertain the 

 exact chemical composition of the living tissues, the next best thins; 

 which can be done is to find out as much as possible about the com- 

 position of the same tissues after they are dead. This is the assistance 

 which the science of Chemistry can afford to the physiologist, and the 

 same science is concerned with the composition of the ingesta and egesta, 

 as well as with that of the fluids of the body. 



Having mastered the structure and composition of the bod}', we are 

 brought face to face with physiology proper, and have to investigate the 

 vital changes which go on in the tissues, the various actions taking place 

 as long as the organism is at work. The subject includes not only the 

 observation of the manifest processes which are continually taking 

 place in the healthy body, but the conditions under which these are 

 brought about, the laws which govern them and their effects. 



We know from our study of biology that the cells of which the tis- 

 sues are composed cannot live without food, both solid and liquid. In 

 a complicated organism like the body of man, the tissues cannot supply 

 themselves with food directly like the amoeba, and so it comes that the 

 various tissues are furnished with what they require by means of n 

 fluid, the blood, which is carried to them in tubes or canals, the blood- 

 vessels, which are distributed to every region of the body. In order that 

 the blood shall reach all parts, the system of vessels in which it is con- 

 tained is supplied with a central pumping organ, the heart. Then we 

 find that as the oxygen, which is one of the requisites of the life of the 

 tissues, and which is carried to the tissues by the blood, is used up, a 

 special means is provided by respiration, or breathing, by means of 

 which the blood is exposed to a new supply of oxygen of the air, which 

 is taken into special organs, the lungs, for the purpose, and which at 

 the same time allows of the elimination of the carbonic anhydride the 

 blood conveys from the tissues. Then again, as the solid food for the 

 tissues cannot be conveyed in the blood in the exact form in which it is 

 introduced into the body, a special and complicated apparatus is pro- 

 vided, that of digestion, by means of which the necessary changes are 

 brought about in the food. The digested food is then absorbed and 

 carried to the blood, either directly with little further change by means 

 of another system of vessels in connection with the blood-vessels, the 

 lymphatic vessels, or after passing through a special organ or gland, the 

 liver, by means of which some further changes take place. In the 

 digestive apparatus we have the organs, the stomach and intestines, into 

 which the food is received for the purpose of being acted upon by cer- 

 tain chemical agents, of which ferments, bodies which are capable of 

 setting up profound changes in other bodies without themselves under- 



