IIlJ THE DIGESTIVE ORGANS 31 



mucous alveoli these cells contain granules the number and 

 position of which vary according to the condition of the gland, 

 whether it is active or resting. The granules contain a substance 

 called mucigen which is the j)recursor of the mucus to be dis- 

 charged. In the resting state they are scattered over the greater 

 part of the cell, the nucleus of which may be seen near one edge. 

 During activity the granules are passed into the lumen j^art of 

 the cell leaving the protoplasm of the cell relatively clear. In 

 the cells of serous glands during rest the granular material is so 

 abundant that the nuclei are occluded, only becoming visible 

 Avhen the gland passes into the active state. The nuclei are 

 placed centrally. In the exhausted state the cells of serous 

 glands retam only very few granules which are left on the inner 

 edge near the lumen, which has grown larger, while the cells have 

 become correspondingly smaller. The granules of serous glands 

 like those of mucous glands represent the precursors of the 

 secretion which is subsequently discharged. This account of the 

 salivary glands is based on the observations of Langley upon the 

 living glands and not merely upon glands which were preserved. 



The salivar}' glands are as a rule mcU developed in herbivorous 

 animals, but the submaxillary and sublingual glands are smaU 

 and inactive m the horse. The parotid is four times the weight 

 of the submaxillary and sublingual in the liorse. The parotid in the 

 horse is about half as heavy agam as in the ox. (Ox283gs. Horse 

 400 gs.) The parotids (horse) secrete seven-tenths of the total 

 salivary secretion. The parotid in all animals secretes more than 

 the submaxillary or sublingual but it is not necessarily the largest 

 in size. The amount of secretion discharged by the saHvary glands 

 depends upon the degree of drjmess in the food, hay absorbing 

 far more saliva than oats, and oats absorbing more than green 

 fodder. Colin has estimated that a horse on an average will 

 secrete 84 lbs. of saliva in a day, and an ox 112 lbs. 



Mixed saliva is a viscid, turbid, colourless fluid with a slightly 

 alkaline reaction. The viscidity is due to the presence of mucin, 

 and the cloudiness to epithelial cells derived from the lining of 

 the mouth, and to the salivary corpuscles. These are small 

 granular cells, and may be altered leucocytes or lymphocytes 

 derived from the tonsils. Saliva contains only about one per 

 cent, of solid constituents altogether, and these include mucin 

 (already mentioned) and other proteins in small quantities, a 



