FOOD DIGESTION AND RESPIRATION 79 



and thumb along. This last kind of movement is known as 

 "peristaltic contraction'' 



Now as to the way in which these movements are produced. 

 It must be assumed that the reader is aware that there are 

 structures called muscles, which have the power of shortening when 

 required. If arranged in the form of a ring, the ring must be 

 narrowed when the muscle shortens, or even closed up entirely. 

 The wall of the intestine consists in part of muscular tissue, which 

 can, by shortening at particular points, empty out the contents of 

 the intestine at these points into the adjoining part. Further, 

 there are two kinds of muscular tissue found in the body, one kind 

 forming the muscles most familiar to us in our arms and legs, and 

 so on. These are at rest, until ordered to activity by the brain 

 (E., p. 195) ; the other kind, which forms a large part of the walls 

 of hollow organs and tubes, such as the heart, intestine, and 

 blood vessels are, when left to themselves, in a state of moderate 

 activity, which shows itself in two ways. It may be present as a 

 steady, uninterrupted state of "contraction," as the shortening of 

 muscle is usually, but incorrectly, called. This state is, as a rule, 

 only partial, since it can either increase or decrease. . Or the 

 spontaneous activity may be shown by a series of contractions and 

 relaxations, following one another at regular intervals. The 

 intestinal muscle, when removed from any connection with the 

 nervous system, manifests both these properties at the same time. 

 The series of contractions starts at a point and passes as a wave 

 from point to point along the course of the intestine. 



It is clear that before any ordered movements, such as those 

 decribed as part of the digestive mechanism, can take place, a 

 means of control of spontaneous activities must be arranged. It is, 

 in point of fact, usual to find that muscular tissue of this kind, 

 which is known by various names, " smooth," " non-striated," or 

 " involuntary," is under the influence of two kinds of nerves, one 

 of which restrains its activity, the other increases it. We do not 

 know yet how the two effects are actually produced, except that 

 they are due to the different ways in which these two kinds of 

 nerve fibres end in the muscle cell itself, not to any peculiarity 

 of the nerve fibre itself or the process in it. The restraining nerves 

 are called "inhibitory," these increasing activity, "excitatory." In 

 the case of the intestine, the excitatory nerve, a part of that nerve 

 known as the vagus, has also inhibitory functions of a certain 

 kind. Its fibres are not connected directly with the muscle fibres, 

 but with a layer of nerve fibres and cells in the wall of the intestine. 

 Secondary fibres are supplied to the muscle itself from this layer. 

 Now we saw that when the digestive process at one place has gone 

 on for a certain time, the backward and forward movements cease 



