Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 385 



in some way or other, by some direct action quite apart from the 

 central nervous system, is able to increase this power so that, 

 without any aid from the central nervous system, as after section 

 of the vagi, adequate peristaltic movements can, under favourable 

 circumstances, be carried out. Nevertheless in the normal course 

 of events satisfactory movements are still further secured by the 

 reflex action through vagus fibres just described. Thus, in the 

 dog, the act of swallowing food or even the mere smell of food 

 has been observed to increase the movements of a piece of intes- 

 tine isolated from the rest of the alimentary canal but retaining 

 its connections with the central nervous system. Under this 

 view the peristaltic movements produced by centrifugal stimu- 

 lation of the vagus in the neck are comparable not so much with 

 the contraction of a skeletal muscle when its motor nerve is stimu- 

 lated as with the beats which may be called forth in an inhib- 

 ited or otherwise quiescent heart by stimulation of the cardiac 

 augmentor fibres. 



Indeed we may perhaps call the vagus fibres which pass to the 

 stomach and intestines augmentor fibres rather than motor fibres. 

 We have all the more reason to do so since there exist companion 

 but antagonistic inhibitory fibres. If while lively peristaltic 

 action is going on in the bowels, the splanchnic nerves be stimu- 

 lated the bowels are brought to rest, often in a very abrupt and 

 marked manner. Inhibitory fibres therefore run in the splanch- 

 nic nerves, Fig. 81, Spl. mag. and min., passing along them 

 from the spinal cord to the abdominal plexuses, and thence to 

 the alimentary canal. 



This view however that the movements of the alimentary 

 canal are of a spontaneous nature, simply augmented on the 

 one hand and inhibited on the other by the central nervous sys- 

 tem, can only be applied to the middle regions, to the stomach 

 and intestines in which peristaltic action is seen in its simple form. 

 At the beginning of the alimentary canal, at the mouth and phar- 

 ynx and also at the oesophagus, the central nervous system inter- 

 venes in a decided manner : the movements of these parts, as we 

 have seen, are carried out directly by the central nervous sys- 

 tem. Something similar is also seen at the end of the canal, at 

 the rectum and sigmoid flexure. These parts are governed on 

 the one hand by fibres reaching them from the lower regions 

 of the cord by the sympathetic system, by the hypogastric nerves 

 and hypogastric plexus, and on the other hand by fibres reaching 

 them along certain cerebro-spinal, namely sacral, nerves (in the 

 dog the second and third sacral nerves) by the branches of these 

 nerves, called nervi erigentes (Fig. 84). And the government by 

 these nerves is one in which the movements are directly carried 

 out by means of the central nervous system. 



Hence this is the part of intestinal movement which fails in 

 diseases of the central nervous system ; the failure leading to 



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