DIGESTION 



161 



in due course at the point from which the lower end was 

 removed. 



It is not uncommon in watching a bolus pass down the neck 

 of the horse to see it suddenly come to a standstill, and then 

 slowly pass on again after probably an attempt to ascend. This 

 is generally due to absence of saliva. In rumination and in 

 vomiting the wave runs upward from the stomach to the 

 pharynx. Division of the vagus interferes with the passage of 

 food along the oesophagus, which in consequence becomes 

 blocked. 



The Saliva. 



During the process of mastication the food becomes mixed 

 in the mouth with a fluid known as saliva, the secretion of which 

 occurs in three distinct pairs of glands. The method by which 

 it is formed is important to understand, as much the same process 

 occurs in other secretory glands which we have not the same 

 opportunity of watching during their activity. 



Classification of Salivary Glands.— The three glands which 

 secrete saliva are the parotid, submaxillary, and sublingual; 

 these are structurally divided into two groups, mucous and 

 serous (or albuminous) glands, the submaxillary and sublingual 

 being types of the first, the parotid the type of the other. 



The following table from Colin shows the relative proportion 

 of the glands in various animals : 



Parotid ' - / 

 Submaxillary / 

 Sublingual / - 



The table shows that, excepting the pig, the horse has the 

 best-developed parotid system, while his submaxillary glands are 

 very small. With the ox the parotid and submaxillary glands 

 are nearly equal in weight. Colin showed that the size of a sali- 

 vary gland does not determine its secretory power ; the parotid 

 of the horse secretes from fifteen to twenty times more saliva 

 than the submaxillary, while it is only about five times heavier. 

 Similarly the parotid of the ox secretes four times more saliva 

 than the submaxillary, though they are nearly of equal weight. 



All the salivary glands belong to the class known as ' compound 

 tubular.' The parotid is regarded as the type of serous (or 



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