404 OF FOOD, AND THE DIGESTIVE PROCESS. 



428. When the food has been propelled downwards by the Pharyngeal muscles, 

 so far as their action extends, its further progress through the (Esophagus is 

 effected by a kind of peristaltic contraction of the muscular coat of the tube itself. 

 This movement is not, however, due only to the direct stimulus of the muscular 

 fibre by the pressure of the food, as it seems to be in the lower part of the ali- 

 mentary canal; for Dr. J. Reid has found, by repeated experiment, that the 

 continuity of the oesophageal branches of the Pneumogastric with the Medulla 

 Oblongata, is necessary for the rapid propulsion of the food; so that it can scarcely 

 be doubted, that an impression made upon the mucous surface of the oesophagus, 

 conveyed by the afferent fibres of these nerves to their ganglionic centre, and 

 reflected downwards along the motor fibres, is the real cause of the muscular con- 

 traction. If the Pneumogastric be divided in the rabbit, on each side, above the 

 oesophageal plexus, but below the pharyngeal branches, and the animal be then 

 fed, it is found that the food is delayed in the oesophagus, which becomes greatly 

 distended. Further, if the lower extremity of the Pneumogastric be irritated, 

 distinct contractions are seen in the oesophageal tube, proceeding from above 

 downwards, and extending over the cardiac extremity of the stomach. We have 

 here, then, a distinct case of reflex action without sensation, occurring as one of 

 the regular associated movements in the natural condition of the animal body ; 

 and it is very interesting to find this following upon a reflex action with sensa- 

 tion (that of the pharynx), and preceding a movement which is altogether uncon- 

 nected with the Spinal Cord (that of the lower part of the alimentary canal). 

 The use of sensation in the former case has been already shown ( 424). The 

 muscular fibres of the oesophagus are also excitable, though usually in a less 

 degree, by direct stimulation ; for it appears that, in some animals (the Dog, for 

 example), section of the pneumogastric does not produce that check to the pro- 

 pulsion of the food, which it occasions in the Rabbit ; and .even in the Rabbit, 

 as Dr. M. Hall has remarked, 1 the simple contractility of the muscular fibre occa- 

 sions a distinct peristaltic movement along the tube, after its nerves have been 

 divided ; causing it to discharge its contents, when cut across. Such a movement, 

 indeed, seems to take place in something of a rhythmical manner (that is, at short 

 and tolerably regular intervals), whilst a meal is being swallowed; but, as the 

 stomach becomes full, the intervals are longer, and the wave-like contractions 

 less frequent. That the action of the Cardiac sphincters is reflex, and is depend- 

 ent upon the " nervous circle" furnished by the Pneumogastric nerves and their 

 ganglionic centres, would appear from the fact that, when the trunks of these 

 nerves are divided, the sphincter no longer contracts, and the food regurgitates 

 into the oesophagus. The re-opening of the cardiac orifices, on pressure from 

 within (which is usually resisted by the sphincter, as in the acts of defecation, 

 parturition, &c.), is one of the first of that series of reversed actions which con- 

 stitutes the .act of Vomiting ( 431); and this is accompanied by a reversed 

 peristaltic action of the oesophagus. The independence of these actions, one of 

 another, and their relation to a common cause, are remarkably shown by the fact 

 that when vomiting takes place as a consequence of the injection of tartar emetic 

 into the veins, the reversed peristaltic action of the oesophagus is performed even 

 after its separation from the stomach. 



429. The food, which, thus propelled along the oesophagus, enters the Stomach 

 through its cardiac orifice in successive waves, is immediately subjected to a 

 peculiar peristaltic movement, which has for its object to produce the thorough 

 intermixture of the gastric fluid with the alimentary mass, and to separate the 

 portion which has been sufficiently reduced, from the remainder. The fasciculi com- 

 posing the muscular wall of the human stomach are so disposed as to lessen its 

 diameter in every direction ; and whilst the cavity is empty, they are uniformly 



1 " Third Memoir on the Nervous System," $ 201. 



