52 READINGS IN BIOLOGICAL SCIENCE 



from desiccation. Our lungs, for instance, are outgrowths of the anterior 

 part of our digestive tube. The finer subdivisions of the bronchial tubes 

 lead to very thin-walled air cells through which respiratory exchange 

 readily occurs between the air and the blood in the capillaries with which 

 the air cells are richly supplied. If our lungs were ironed out, so to speak, 

 the total area of their surface in intimate association with the blood would 

 be about equal to the wall space of a fair-sized room. 



In organs devoted to absorbing food the same principle is abundantly 

 illustrated. Consider the surface of a large tree with its numerous leaves 

 having expanse which may be more than an acre in area. In this expanse of 

 leaves the carbon dioxide of the air is absorbed and, together with water, is 

 built up into carbohydrates under the influence of sunlight. And in the 

 root system with its millions of root hairs there is a great expanse of sur- 

 face through which water and salts are absorbed from the soil. Organs 

 of excretion, such as our kidneys and their numerous coiled tubules, are 

 devices for bringing a large area of excretory cells into close relationship 

 with the blood. The same statement applies to glands of all sorts, whether 

 devoted to the elaboration of digestive juices or the production of other 

 substances. If we survey our own bodily structure or that of any other 

 complex animal and consider how much of its make-up consists of exten- 

 sions of surfaces involved in absorption, secretion, excretion and respira- 

 tion, we will find that we have included no small part of its structural com- 

 plexity. 



But the story by no means ends here. In order to live at all every organ- 

 ism, even the simplest, must perform the basic functions of absorption, 

 assimilation, respiration, excretion, conduction and reproduction. But in 

 order that these basic functions can be discharged in a more highly de- 

 veloped organism, other activities subservient to them have been super- 

 added. Let me illustrate. All organisms must take in nutriment from the 

 outside. In animals the food usually requires to be digested before it can 

 be absorbed and gain access to the living protoplasm. The essential feature 

 of digestion is splitting up food substances by means of enzymes, or fer- 

 ments until they are rendered capable of solution and diffusion through 

 living membranes. Digestion is a process subsidiary to absorption. The 

 amoeba performs this function in little vacuoles in its protoplasm formed 

 by the secretion of fluid around engulfed particles of food. These vacuoles 

 disappear after their work is accomplished, and the undigested residue of 

 the food is expelled to the outside. They are little stomachs improvised for 

 the occasion. In a hydra we have permanent specialized organs set apart 

 for the function of digestion, but the structures involved are of a very 

 simple and primitive kind. In striking contrast with this is our own diges- 

 tive machinery with its comphcated stomach and intestine and the highly 

 developed glands of liver and pancreas, to say nothing of numerous small 

 glands elaborating their specific kinds of digestive ferments. But where 



