;8 INTRODUCTION TO GENERAL PHYSIOLOGY 



of bacteria. In the course of the changes brought about by these 

 organisms, intermediate products, such as glucose from the cellulose 

 of the plant tissues, are formed, and these are to a large extent 

 absorbed before they have undergone further destruction. 



Movements of the Alimentary Canal 



The various digestive enzymes, whose action has been briefly 

 described above, are formed either by cells lining little pits on the 

 wall of the cavities or by separate organs called glands, which pour 

 in their products by means of ducts. The process of formation 

 of these juices is known as "secretion? to which we shall return 

 presently. 



But it will be clear that the effective mixing of these secretions 

 with the food requires a process similar to the kneading of dough 

 to make bread. Moreover, the absorption of the digested material 

 needs the bringing into contact with the wall of the intestine of 

 all parts of the mass in turn. And again, the useless indigestible 

 portion must be carried along to make room for the new material 

 arriving from the stomach, while the new material itself needs 

 exposing to the action of different parts in turn. 



The mechanism by which these things are done will be better 

 understood after the later portions of this book have been read. 

 The process is a complex one, involving nervous and muscular 

 factors. It will be best described in its essential features by taking 

 the case of the small intestine, where the simple tubular form shows 

 it in a way most easily followed. 



Let us first see what actually happens, and then attempt to 

 explain how it is brought about. We will suppose that an animal 

 has been fed with a meal containing an insoluble powder which is 

 opaque to X-rays, such as bismuth subnitrate. The shadows of 

 the food masses can then be observed on a fluorescent screen in 

 the usual way and their movements followed. Looking at the 

 shadow of a loop which happens for a moment to be at rest, we 

 notice that it is filled with a long column of regular diameter like 

 a sausage. Presently this column is nipped together in several 

 places at the same time, and divided up into a number of small 

 portions. Each of these is next divided up and the parts forced in 

 both directions, so as to join with similar parts of neighbouring 

 masses. The process is repeated many times, and then a powerful 

 nip starts at the end of the mass next the stomach, forcing the 

 whole onwards into the intestine beyond, which becomes lax in 

 order to receive it. This nipping passes along the mass from end 

 to end and empties the loop, much as if it had been seized by the 

 finger "and thumb, and the contents pressed out by sliding the finger 



