368 PRINCIPLES OF GENERAL PHYSIOLOGY 



brought about by direct action on the nerve plexus. Cannon (191L') suggests 

 that a better name for the local reflex in question would be the " mventei ic 

 reflex," and the name has the advantage of being applicable to the whole 

 alimentary canal. 



It seems clear, however, that some further regulation of the passage of food 

 is required to prevent its being hurried along too rapidly ; even a means of 

 sending it backwards and forwards. The muscular contractions causing movement 

 onwards are generally known as "peristalsis" and those in the opposite direct inn, 

 " antiperistalsis," but Cannon (1912) suggests, as a better terminology, the name 

 " diastalsis " for the forward moving wave, controlled by the myenteric reflex 

 and preceded by inhibition ; and " ana- and cata-stalsis " for the rhythmic waves of 

 upward or downward movement, which are present independently of the myenteric 

 reflex and not preceded by a wave of inhibition. These movements take place 

 when the myenteric reflex is put out of action, as it can be by the influence of 

 the central nervous system, as we shall now see. 



There are two great nerves which control nearly the whole of the alimentary 

 canal ; the vagus, which is the ancestral motor nerve for the whole of the gut, 

 except the two extreme ends; even in the higher mammals it continues to act 

 as motor nerve to the large intestine, remarkable as it may seem that a cranial 

 nerve should have so posterior a distribution. Gaskell, in fact (1908, pp. 446-404), 

 draws interesting conclusions as to the origin of vertebrates from the innervation 

 of the alimentary canal. 



Eduard Weber (1846) in his classical article in which he describes the discovery of the 

 inhibitory action of the vagus nerve on the heart, also (p. 49) describes a remnrk;il>le died 

 of the same nerve in the -Tench, where it produces a quick contraction of the intestinal 

 muscle, "like skeletal muscle." 



The second great nerve supply is from the sympathetic system and contained 

 in the splanchnic nerves. This is inhibitory. In Fig. 9G a tracing is given which 

 shows how the rhythmic contractions are stopped and the tonus abolished when 

 this nerve is excited. It is, as a rule, impossible to obtain the myenteric reflex 

 unless the splanchnic nerves are cut, owing to the inhibitory control they have 

 over its manifestation. 



A curious fact in connection with the vagus is that its motor effect on the 

 small intestine is preceded by an inhibitory one, and that the motor effect is not 

 shown until after the nerve has been subjected to a series of periods of excitation. 

 The tracing given in Fig. 97 is the result of the ninth of such a series, the first 

 five of which showed no motor action. 



The more posterior part of the large intestine has its motor nerve supply from 

 the pelvic visceral, or autonomic, nerves. 



The best means of investigating the normal movements of the alimentary 

 canal in digestion is that introduced by Cannon (1897 and 1902). The animal, 

 or man, is given bismuth subnitrate, an insoluble, inert powder, mixed with the 

 food ; by this means the contents of the alimentary canal are made opaque to the 

 Rontgen rays and their shadows can be watched on the fluorescent screen. It 

 has been made out that the food, having arrived into a particular section, say the 

 stomach, is imprisoned therein for a time by closure of the sphincters. During 

 this imprisonment it is thoroughly churned, backwards and forwards, until the 

 enzymes have had time for their work, and the products of their activity, if 

 absorbed in this part, have been taken into the blood and lymph. The proee-.-, 

 of churning, whose mechanism can be observed best in the small intestine, <!<>e> 

 not consist in a true antiperistalsis, preceded by a wave of inhibition, but by 

 a series of local contractions dividing the contents into separate masses at different 

 places in turn and thus pushing them upwards and downwards. 



It has been shown by Serdjukov (see Pavlov, 1901, p. 187) that the pyloric 

 sphincter of the stomach opens at intervals to allow a portion of the contents 

 to escape into the duodenum ; as soon as acid is present therein, the pylorus 

 closes by a nervous reflex, so that only a small amount of food is allowed to 

 enter the duodenum at pne time. 



These movements of churning and so on appear to be mainly controlled from 





