314 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



in these animals. Hence, if the bolus of food is larger than three or four 

 centimeters in diameter it is apt to be arrested. In the ox boluses double 

 the size pass without difficulty. 



When once the alimentary bolus is within the grasp of the muscles 

 of the oesophagus it moves onward with considerable force. Mosso, 

 in his experiments made on the dog, found that even when the bolus 

 of food was held back by a weight of four hundred and fifty grammes 

 deglutition was not interfered with. When once the bolus of food 

 reaches the upper part of the oesophagus the pharynx falls, and the bolus 

 traverses the length of the oesophagus under the influence of the succes- 

 sive contractions of the circular and longitudinal muscular fibres. The 

 longitudinal fibres contract first, and draw up the oesophagus to meet the 

 advancing bolus, which is pushed down by the contractions of the annular 

 fibres behind it. Gravity is entirely without influence on the motions of 

 deglutition, as swallowing occurs equally well even when the head is on 

 a lower plane than the entrance of the oesophagus into the stomach. 



The third stage of deglutition is involuntary and is unattended by 

 sensation, though pain may be intense when too large a bolus or a hard, 

 irregular mass is swallowed ; so, also, very hot or very cold substances 

 may be recognized in their passage through the oesophagus by the sensa- 

 tions which they occasion. As a rule, however, the passage of food 

 through the oesophagus is entirely unattended by any feeling. Even 

 acids cause but little sensation. 



That deglutition may be accomplished, it is essential that there 

 must be something to be swallowed. When the mouth contains saliva 

 alone the motions of deglutition may be made, but as the quantit}' of 

 fluid in the mouth decreases deglutition becomes more and more difficult, 

 until finally it is impossible. This fact indicates the reflex nature of 

 the motion of deglutition. As before pointed out, a reflex action re- 

 quires the presence of a stimulus, its conduction to a nerve-centre, and 

 the transmission of motor impulse through efferent nerves to a muscular 

 fibre. The stimulus for deglutition is found in the contact of food with 

 the mucous membrane of the mouth, pharynx, and oesophagus. The 

 sensory nerves come from the trigeminal, the glossopharyngeal, and the 

 superior laryngeal nerves. Excitation of any of these nerves produces 

 movements of deglutition. 



In the case of the oesophagus the pneumogastric is the sensory 

 nerve. The centre of the movements of deglutition is found in the 

 medulla oblongata. The motor nerves are the glossopharyngeal, sup- 

 plying the muscles of the pharynx; the hypo-glossal, supplying the 

 muscles of the tongue ; the trigeminal and facial, supplying the muscles 

 of mastication' and the pneumogastric, supplying the muscles of the 

 larj'nx and oesophagus. 



