260 OF FOOD AND THE DIGESTIVE PROCESS. 



455. The progress of the food along the (Esophagus is aided by 

 the action of the muscular coat peculiar to it. This is composed of 

 the non-striated fibre ; and, like that of the intestinal canal further on, 

 it is usually stimulated to contraction by the direct contact of the 

 stimulus, and not either by the will, or by the reflex action of the spinal 

 cord. The movement produced by it is of the peristaltic or wave-like 

 kind ; the contractions being limited to one portion of the tube, and 

 being propagated along it from above downwards. This action con- 

 tinues after the division of all the nerves supplying the oesophagus ; 

 and it cannot, therefore, be dependent upon the brain or spinal cord. 

 It may be observed to take place in a rhythmical manner (that is, at 

 short and tolerably regular intervals), whilst a meal is being swallowed ; 

 b'ut as the stomach becomes full, the intervals are longer and the 

 wave-like contractions less frequent. The degree in which the action 

 of the oesophagus alone, without that of the surrounding muscles, is 

 capable of propelling the food into the stomach, seems to vary in diffe- 

 rent animals. When the latter are paralysed in the Dog, by section of 

 the nerves that supply them, the food that has entered the oesophagus is 

 still propelled into the stomach ; but this is not the case in the Rab- 

 bit, the action of its cesophageal fibres not being sufficient to carry the 

 food onwards to the stomach, though it will expel it from the divided 

 extremity of the tube when it is cut across. The usual peristaltic 

 movements of the oesophagus are reversed in Vomiting : and this rever- 

 sion has been observed, even after the separation of the stomach from 

 the oesophagus, as a consequence of the injection of tartar emetic into 

 the veins. 



456. At the point where the oesophagus enters the Stomach, the 

 cardiac orifice of the latter, there is a sort of sphincter, or circular 

 muscle, which is usually closed. This opens when there is a sufficient 

 pressure on it, made by the accumulated food propelled by the move- 

 ments of the oesophagus above ; and it then closes again, so as to retain 

 the food in the stomach. The closure is due to reflex action ; for when 

 the nerves supplying it are divided, the sphincter no longer contracts, 

 and the food regurgitates into the oesophagus. The opening of the car- 

 diac orifice is one of the first acts which takes place in vomiting. 



457. In Ruminating animals, there is a very remarkable conforma- 



Fig. 77. 



Compound Stomach of Sheep : , oesophagus ; 6, paunch ; c, second, or honeycomb stomach ; 

 d, third stomach, or many-plies; e, fourth stomach or reed; /, pylorus. 



tion at the lower end of the oesophagus, which is destined to regulate 

 the passage of food into the different compartments of the stomach, 



