206 DIGESTION". 



glands, the follicular glands of the tongue and general mucous surface, and certain 

 glandular structures in the mucous membrane of the pharynx. The liquid which be- 

 comes more or less incorporated with the food before it descends to the stomach, and 

 which must be considered as the digestive fluid of the mouth, is known as the mixed 

 saliva; but the study of the composition and properties of this fluid as a whole should be 

 prefaced by a consideration of the diiferent secretions of which it is composed. 



The salivary glands belong to the variety of glands called racemose. They closely 

 resemble the other glands belonging to this class, and their structure will be considered 

 more particularly under the head of secretion. 



Parotid Saliva. The parotid is the largest of the three salivary glands. It is sit- 

 uated below and in front of the ear and opens by the duct of Steno into the mouth, at 

 about the middle of the cheek. The papilla which marks the orifice of the duct is 

 situated opposite the second large molar tooth of the upper jaw. 



Numerous opportunities have presented themselves, in cases of salivary fistula, for the 

 study of the properties of the pure parotid saliva in the human subject ; and the situation 

 of the duct of Steno, in the herbivora especially, is such that this fluid can easily be ob- 

 tained by operations on the inferior animals. Prof. J. 0. Dalton has obtained the pure 

 parotid saliva from the human subject by simply introducing a silver tube, of from -fa 

 to -fa of an inch in diameter, into the duct by its opening into the mouth. 



The following facts with regard to the properties of the parotid saliva observed by 

 Dalton are given in his own words, in a communication kindly made in answer to certain 

 inquiries : 



" On the 28tb of July, 1863, I obtained, from a strong, healthy man, about two 

 drachms of the mixed saliva of the mouth, by causing him to hold in his mouth for a 

 short time a clean glass stopper, and collecting the secretion as it was discharged. 



" One hour afterward I obtained, from the same man, four drachms of pure parotid 

 saliva, by introducing a long silver canula into the natural orifice of Steno's duct, on the 

 left side, and collecting the saliva as it flowed from the outer extremity of the canula. 



"The two kinds of saliva compared as follows: 



" Both were distinctly alkaline in reaction ; the parotid saliva rather the more so. 



" The parotid saliva was rather clear and watery in appearance ; the saliva of the mouth 

 was quite opaline, with admixture of buccal epithelium, but became clear on filtration. 



"The parotid saliva was rendered turbid by the action of heat, and by the addition of 

 nitric acid, as well as sulphate of soda in excess ; but not by sulphate of magnesia, nor by 

 ferro-cyanide of potassium with acetic acid. 



" The saliva of the mouth, filtered clear, became turbid by heat and by nitric acid, 

 but showed no precipitate by either sulphate of soda or sulphate of magnesia in excess. 

 There was also a slight precipitate on the addition of pure acetic acid, which did not take 

 place in the parotid saliva, 



" The parotid saliva showed no traces of sulpho-cyanogen on the addition of the per- 

 chloride of iron, but they were distinctly marked in the buccal saliva. 



" On mixing the two kinds of saliva with boiled starch, and keeping the mixture at 

 the temperature of 100 Fahr., sugar was present in both specimens at the end of five 

 minutes. There was no marked difference between them in this respect. 



" While making some similar experiments to the above on a previous patient, in April, 

 1863, I found that with the canula introduced into Steno's duct, not only was the dis- 

 charge of parotid saliva increased by the mastication of food, but that it ran from the 

 canula very much faster than in a state of rest, whenever the patient smiled, spoke, or 

 moved his lips or cheeks in any way." 



The organic matter of the parotid saliva is coagulable by heat (212 Fahr.), alcohol," 

 and the strong mineral acids. Dalton found, in the human saliva, that it was also coagu- 

 lated by an excess of sulphate of soda ; but Bernard states that, in the parotid saliva of 



