Notvvithstanding that the subject has been laboriously investigated by some of the most intrenious experimenters of the-day, the uses of6the saliva in the economy are evidently still but imperfectly ascertained. Spallanzani was inclined to believe in a solvent action which this fluid was capable of exerting on animal matters, and thought that food, when inclosed in a tube perforated with numerous small holes, and placed in saliva, was more ra pidly broken up and dissolved than when water only was used. The farther observations of Berzelius and Mitller tended, however, to im pugn the correctness of this opinion, pure water acting, according to their experiments, quite as efficiently as saliva.
Some experitnents have been made by Hiinefield, by which he thinks he has shown saliva to possess a peculiar action on fibrin : this, however, requires confirmation. In the year 1831 Leuchs made a most important discovery in connection with the history of saliva, viz., that when boiled starch is added to it, and the mixture is kept at a temperature of 98°, the starch becomes converted into sugar. This action has been since investi gated by Mialhe, who attributes the pheno menon to the presence of a peculiar proximate principle existing in saliva, to which lie has given the name of animal diastase, in con sequence of its possessing the qualities of that principle as it exists in the vegetable kingdom, in germinating seeds. In order to obtain this substance the saliva is to be fil tered, and then precipitated by the addition of absolute alcohol, of which generally from five to six times the weight or the saliva are re quired to effect the purpose. The animal diastase falls in the form of a flocculent pre cipitate, which may be collected and dried on a filter. It forms about 0-2 of the whole saliva. It is a white substance, insoluble in alcohol.
A series of experiments have been lately made by M. Bernard, with a view of deter mining what the action of saliva may be in the digestive process.
He first satisfied himself that the saliva of the horse and the dog, as well as that from the human subject, possessed the property of de composing starch into sugar, under the con ditionsof temperature above described. The saliva of the dog, however, effected the con version but slowly, that of the horse more quickly, but neither nearly with the rapidity of human saliva. The dog's saliva required nearly eight times as long as that from man, and that of the horse nearly four times as long. Care was taken in these experiments to employ the same quantities of saliva and of starch.
Pure saliva, obtained from the parotid and submaxillary glands of the dog, were found by Bernard quite incompetent to effect the transformation of starch. This agrees with the observation of Lassaigne, who found that pure saliva froin the parotid of horses pos sessed no transforming power of the kind, though mixed saliva taken from the cesophagus acted well ' on starch. According to Ber nard's experhnents, the explanation of this rests on the fact that the power of trans formation is a property of the secretion from the mucous membrane lining the mouth, for on placing layers of that membrane in contact either with starch or sugar he obtained de composition, and lactic acid was produced.
He thus reduces the importance of saliva, as an adjunct in digestion, to little thore than that of a lubricating fluid.
Saliva of Anintals.— The saliva of animals has not been much experimented upon. Berzelius remarks as follows on the saliva of the dog it " As obtained from the pa rotid, it is a pale yellow fluid of mucila ginous consistence, resembling white of egg in its physical characters. It leaves 2.58 per cent. of solid matters on evaporation. These solids form a transparent pale yellow varnish on the surface of the evaporating dish, which becomes moist by exposure to air. Alcohol extracts principally chlo ride of sodium from this mass, and, by eva porating the alcoholic solution, crystals of the chloride can be obtained nearly' in a pure state, being, however, mixed with a small proportion of a yellowish substance, com posed principally of lactate of soda and os mazome. Sulphocyanogen cannot be de tected with certainty in the alcoholic extract, and but a trace only of its reaction with the salts of iron can be observed.
The portion of solid matter which is in soluble in alcohol contains salivary matter, combined with soda, and its reactions accord perfectly with those of the salivary matter found by Gmelin in human saliva. Phosphate of potash, phosphate of soda, and a small proportion of carbonate of lime, also exist in this saliva.
The saliva of the sheep, according to Ber zelius, is clear, and not adhesive, like that of the dog. It has a feeble saline taste, and a faint alkaline reaction. When dried, it leaves P68 per cent. of solid matter, which forms an opaque white membrane, and becomes rnoist by exposure. Chloride of sodium is extracted from this mass, in octahedral crystals, by di gestion with alcohol. The salts of iron yield ample evidence of the presence of sulpho cyanogen in the alcoholic solution. The por tion of solid matter insoluble in alcohol, when treated with water, yields little else than salts. So completely is this the case, that the eva porated aqueous solution scarcely gives out empyreumatic odour while being heated to redness. The mass, which is insoluble both in water and alcohol, is brittle and mem branous, insoluble in acetic acid, and not gelatinised when moistened by it. The acid, however, dissolves out phosphate of lime, after which it is precipitable by the addition either of ammonia or oxalate of lime, but not by in fusion of galls.
quantity. The sulphocyanide which exists in the saliva of man and of the sheep cannot be satisfactorily detected in the dog. Ptyalin is almost wanting in the saliva of the sheep, while that of the dog is deficient in animal extractive matter.
Lassaigne and Leuret found the same quan tity, viz. about one per cent. of solid matters, in the saliva of man, the horse, and the dog.
The saliva of insects has been collected by Reuzzer #, but not in quantity to admit of analysis. It was found, however, to yield an alkaline reaction.