304 THE TISSUES AND MECHANISMS OF DIGESTION. 



the alimentary canal but retaining its connections with the central nervous 

 system. . Under this view the peristaltic movements produced by centrifugal 

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

 contraction of a skeletal muscle when its motor nerve is stimulated as with 

 the beats which may be called forth in an inhibited 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 (and these, we may remark, are, like the cardiac augmentor 

 fibres, non-medu Hated fibres along the greater part of their course) aug- 

 mentor 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 

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

 marked manner. Inhibitory fibres, therefore, run in the splanchnic nerves 

 (Fig. 89, Spl. maj, and mm.), passing along them from the spinal cord to 

 the abdominal plexuses, and thence to the alimentary canal. 



It will be noticed that the splanchnic nerves, while containing vaso-con- 

 strictor, i. e., augmentor, fibres for the bloodvessels of the intestines, carry 

 inhibitory fibres for the muscular coat; and probably the vagus, while con- 

 taining augmentor fibres for the muscular coat, carries inhibitory dilator 

 fibres for the bloodvessels. It may further be remarked that the vagus, 

 while supplying augmentor fibres for the muscular mechanisms of the ali- 

 mentary canal, carries, as we so well know, inhibitory fibres for the cardiac 

 muscular mechanism. 



In the above statement we have purposely used the general term, peri- 

 staltic movement ; but, as we have seen, in the movements of the alimentary 

 canal, two sets of muscles are concerned the circular and the longitudinal. 

 Now, in the rectum we are able to recognize that the two sets of muscles 

 have quite distinct nervous supplies. The longitudinal coat is governed 

 by nerve-fibres which, in the dog, leave the spinal cord in the anterior roots 

 of the second and third sacral nerves (Fig. 89, S. 2, S. 3), pass along the 

 branches of those nerves frequently spoken of as the nervi erigentes, I. r., to 

 the hypogastric plexus (pi. hyp.), and thence to the rectum. Stimulation 

 of these nerves causes contractions of the rectum, which are confined to 

 the longitudinal coat and, as we have said, pull the rectum down. The cir- 

 cular coat is governed by fibres which leave the spinal cord by the anterior 

 roots of the lower dorsal and first two lumbar nerves, Fig. 89 (coming from 

 the lower part of that spinal region from which, as we have seen ( 155) 

 the vaso-constrictor fibres take origin), and, early losing their medulla, pass 

 to the rectum by the inferior mesenteric ganglia, the hypogastric nerves, 

 and hypogastric plexus (Fig. 89, m. gl., n. hyp., pi. hyp.). Stimulation of 

 these fibres gives rise to contractions which are confined to the circular coat 

 and squeeze out the contents of the rectum. A similar double nervous 

 supply probably governs the longitudinal and circular coats along the 

 whole alimentary canal ; but the details of such a supply are at present 

 unknown. 



Our knowledge, moreover, concerning the details of any special nervous 

 mechanisms, by means of which the more complicated movements of the 

 stomach, including the closing and opening of the sphincters, are carried out, 

 is at present very imperfect. We cannot add to what we have incidentally 

 said in speaking of vomiting. 



The movements of the rectum, including the sigmoid flexure, appear 

 to be much more closely dependent on the central nervous system than 

 are those of the rest of the alimentary canal. As we have said, the move- 

 ments of both large and small intestine are rather assisted and aug- 



