8o INTRODUCTION TO GENERAL PHYSIOLOGY 



and a wave of contraction passes along, preceded by a wave of 

 relaxation. This peristaltic wave occurs also in the intestine after 

 removal from the body, if distended locally in any way. The most 

 satisfactory way of explaining the fact, which has been called the 

 " law of the intestine," is that the nerve cells with which the vagus 

 fibres join give off, each one, two nerve fibres, one of which goes 

 backwards to supply muscle nearer the stomach, and is excitatory ; 

 the other goes forward, and is inhibitory. When, therefore, some- 

 thing happens in the interior of the intestine opposite one of these 

 cells, of such a nature as to set this cell into activity, a contraction 

 will be started behind the place in question and a relaxation in 

 front, both these processes travelling onwards as the food mass is 

 pushed between them. We must suppose that something, perhaps 

 of a chemical nature, happens at a certain stage of digestion, and 

 that the movement is brought about which sends the mass onwards. 



One of the properties of smooth muscle is that it is stimulated 

 to contraction by being stretched. We understand, therefore, how 

 the distension of an intestinal loop by food passing into it will 

 result in contraction, at this place, and that when a contraction 

 occurs over the food mass, this will be forced into a neighbouring 

 region and in turn cause contraction here. But it is not quite so 

 easy to explain why a number of points along a column of food 

 material enter into activity at the same time, as we saw happens. 

 It is possible that small differences in degree of distension may 

 determine where a contraction is to take place. But we must also 

 remember that any of the activities described can be stopped by 

 the inhibitory nerve fibres in order that a different kind of activity 

 may take its place. These fibres are contained in the nerves called 

 " splanchnic," which issue from the central nervous system, so that 

 it is not impossible that there may be control of co-ordinate move- 

 ments by the higher centres in response to messages received. 

 The inhibitory fibres in the splanchnic nerves are readily set into 

 activity by stimulation of nerve fibres in the intestine which carry 

 messages to the nerve centres, producing what we shall learn later 

 is a " reflex action." Thus, injury to the intestine, as by gun-shot 

 or shell wound, results in a long-lasting cessation of movement, a 

 reflex "paralysis" of the muscular wall, clearly a beneficial process, 

 especially if the injury has resulted in perforation of the wall of 

 the intestine. 



The feeling of hunger^ as distinct from appetite, is caused by 

 contractions of the empty stomach, by which nervous structures 

 are pressed upon and stimulated. 



Appetite^ on the other hand, may be described as pleasure 

 anticipated from the taste of food, and may, of course, be present, 

 independently of hunger or actual need for food. It is, however, 



