132 OF FOOD, AND THE DIGESTIVE PROCESS. 



dulla Oblongata is necessary for the rapid propulsion of the food ; from 

 whence it appears 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 contraction. The following are the general results of Valentin, 1 

 so far as they apply to this part of the subject: The lower part of the 

 oesophagus in the neck is made to contract peristaltically from above down- 

 wards, by irritation of the roots of the first three cervical Spinal nerves, and 

 of the cervical portion of the Sympathetic, through which last the former 

 evidently operate. The thoracic portion of the oesophagus is made to con- 

 tract, by irritation of the lowest Sympathetic ganglion of the neck, and of 

 the higher thoracic ganglia, and also of the roots of the lower cervical 

 Spinal nerves. Muscular contractions of the Stomach are produced in the 

 rabbit, by irritation of the roots of the 4th, 5th, 6th, and 7th cervical Spinal 

 nerves, and of the 1st thoracic, so that a distinct furrow is evident between 

 the cardiac and pyloric portions of the viscus; and the lower the nerve is 

 irritated, the nearer to the pylorus do the contractions extend. If the 

 Pneumogastric be divided in the rabbit, on each side, above the cesophageal 

 plexus, but below the pharyngeal branches, and the animal be then fed, it 

 is found that the food is delayed in the oesophagus, from spasmodic contrac- 

 tion of the cardiac sphincter, which becomes greatly distended. Further, if 

 the lower extremity of the Pneumogastric be irritated, distinct contractions 

 are seen in the cesophageal 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 

 sensation (that of the pharynx), and preceding a movement which is but 

 slightly connected with the Spinal Cord (that of the lower part of the ali-' 

 mentary canal). The use of sensation in the former case has been already 

 shown. The muscular fibres of the (Esophagus 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 pueumogastric does not pro- 

 duce that check to the propulsion of the food, which it occasions in the 

 Eabbit; and even in the Rabbit, as Dr. M. Hall has remarked, 2 the simple 

 contractility of the muscular fibre occasions 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 wavelike contractions less frequent. 

 The reopening of the cardiac orifice, on pressure from within (which is 

 usually resisted by the sphincter, as in the acts of defecation, parturition, 

 etc.), is one of the first of that series of reversed actions which constitutes 

 the act of Vomiting ; 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, is 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. 



91. The food which, thus propelled along the u'sophagus, enters the Stom- 

 ach through its cardiac orifice in successive waves, is immediately subjected 



1 



1 Valentin, Do Funct. Norv. Cereb. et Ncrv. Symputh., buok ii, chap. ii. 



2 Third Memoir on the Nervous System, \ 201. 



