Ill] THE DIGESTH^E ORGANS 29 



of muscular contraction ^^'hich pass through its entire length 

 from the pharynx to the stomach. Swallowing is facilitated bj- 

 the presence of saliva without which it can only be performed 

 with dithculty. It can be performed against gravity since it is 

 a definite muscular act, the peristaltic waves of contraction j)ass- 

 ing along the oesophagus altogether irrespectively of the position 

 of the individual. In swallowing fluids the action is very rapid 

 and may be carried on at the rate of one swallow every second. 



The first act in the process of swallowing is voluntar}^ but 

 the second and third acts are purely refiex, being presided 

 over by a definite centre in the hind brain which is informed 

 of the presence of food b}^ afferent nerves coming from the 

 pharynx. Impulses are then conveyed by efferent nerves pass- 

 ing to the different muscles concerned in swallowing. Afferent 

 impulses may be started by touching the inside of the windpipe 

 or the rim of the glottis and the swallowing centre will be thereby 

 stimulated. 



The oesophagus ui the horse differs from that of the ox and 

 most other mammals in one important resj)ect. The thin red 

 striated muscle, which surrounds it throughout the greater part 

 of its length, gives place to thick, pale, non-striated muscle at its 

 lower end, where it is tightly contracted, the lumen being ver}- 

 narrow. It is largely for this reason that horses experience such 

 difficulty in vomiting. In the ox and sheep the muscles of the 

 oesophageal wall are red and striated throughout, and the lumen 

 of the organ is widely distended at the lower end, so as easily 

 to admit of the food passing back mto the mouth in the j^rocess 

 of rumination. 



The Salivary Glands. Just prior to and during mastication 

 saliva is secreted in considerable quantities and is poured into 

 the mouth where it mixes with the food. It is secreted in well- 

 developed racemose glands, situated a short distance from the 

 cavity of the mouth, into which the secretion passes bj' ducts 

 leading away from each gland. "There are three jDairs of salivary 

 glands. (1) The parotid glands, so called because they lie in 

 front of each ear. Their ducts communicate with the buccal 

 cavity just opposite the upper molar teeth on each side. These 

 are the glands which in man become characteristically swollen 

 in the disease known as 'mu)nps,' though the other salivary 

 glands are also generally affected in this complaint. (2) The sub- 



