OP.OAXS OF SECRETION.] 



CHEMISTRY. 



333 



13. In fresh tissues, fatty substances are only pre- 

 sent in the form of neutral fats, as oleine and margarine. 

 There is scarcely any part of the body from which 

 these neutral fats are entirely absent. There is, how- 

 ever, very little fat in the lungs, and less in the teeth. 

 It exists in great abundance under the skin ; in the 

 bones as marrow; in the mesentery, by which the intes- 

 tines in the abdominal cavity are, as it were, suspended ; 

 in the orbit of the eye ; and in the female breasts. 

 The smoothness and roundness of the forms of females 

 and children are chiefly attributable to the layers of fat, 

 which cause the soft convexities of their skin. This fat 

 is much less abundant under the skin of men ; and the 

 forms, therefore, of their bones and muscles are much 

 more strongly denned. 



Oleine and margarine are mingled in very different 

 proportions in different parts of the body; and mar- 

 garine coagulates more readily than oleine. It follows, 

 that the greater the predominance of oleine and mar- 

 garine in any part of our body, the greater also will be 

 the lubricity and fluidity of the fat ; as, for instance, 

 in the marrow of the bones, and in the fat under the 

 skin. The firmer fat again, which, for instance, sur- 

 rounds the kidneys, contains more margarine than oleine. 

 We find the bile-fat of the blood unchanged in the 

 brain ; but the latter contains, besides this, oleine, mar- 

 garine, and another kind of fat, in which carbon, hydro- 

 gen, and oxygen, are combined with nitrogen and phos- 

 phorus. Without this peculiar fat the brain cannot 

 exist. It is remarkable that this brain-fat has been 

 detected not only in the blood, but also in the yolk 



14. Sugar is a regular constituent of the liver; 

 the muscles contain also a peculiar kind of sugar, called 

 sugar of the muscles. Lactic acid a substance soluble 

 in water, and compounded of carbon, hydrogen, and 

 oxygen, exactly in the same proportion as in sugar of 

 grapes is found in the muscles in considerable quan- 

 tities. In the brief description of digestion, we saw 

 that, by the influence of the bile, lactic acid is formed 

 from sugar. It is more than probable, that, in the 

 muscles also, lactic acid is produced from their peculiar 

 sir.'ar ; in short, the constituents of fat are also present 

 in the different tissues, though in a very small quantity. 



15. Organs of Secretion. The sum of the processes 

 which we have described in a former section, in order to 

 explain the origin of the solid constituents of our body, 

 is designated by physiologists as nutrition in the strictest 

 sense. Not all matters, however, which transude through 

 the capillaries from the blood are to bo considered as 

 having a direct share in the origination of the solid con- 

 stituents. A very great proportion of the transuded 

 matters does not contribute to the nutrition of the tissues, 

 but remain as a liquid ; which, having been attracted or 

 prepared by certain organs from the blood, is collected 

 into receptacles, and out of these discharged into certain 

 cavities of the body, or altogether out of it. These 

 receptacles are often simply canals, called secretory ducts 

 of the organs; whilst the organs themselves are called 

 glands. In other cases, the secretory ducts lead, before 

 they discharge the liquid, into wider reservoirs, called 

 bladders. Thus we have a seminal bladder, a gall 

 bladder, and a urinary bladder. 



The fluids, which are attracted and collected from the 

 blood by the glands, are, in some respects, of a special 

 and immediate use in relation to the body, by assisting 

 the functions of reproduction and digestion. All fluids 

 which co-operate for these two essential functions, in 

 preserving the species and the individual, are included 

 under the general head of secretions. In all secretory 

 fluids there is contained more or less water, which is a 

 necessary condition of trausudation generally. 



5 16. We begin with the ovum, not only on account of 

 its importance in the preservation of the species, but 

 because, next to the tissues, it is the firmest of the 

 products of secretion. It forms, therefore, a kind of 

 transition from the tissues themselves to the secretions. 

 As the human ovum, in the earliest state of its develop- 

 ment, can only be perceived by a high magnifying power, 



its investigation has been incomplete. We know cer- 

 tainly only this, that it consists of an albuminous sub- 

 stance, combined with oleiue, margarine, brain-fat con- 

 taining phosphorus, bile-fat, sugar, and the inorganic 

 constituents of the blood. 



17. The seminal fluid can also be subjected only to 

 a very imperfect analysis ; but its chief constituent, the 

 seminal principle (spermatine), is, in its essential pro- 

 perties, so nearly analogous to albuminous substances, 

 that we may, without hesitation, classify it with them. 

 Should future investigations confirm the slight distinc- 

 tion already observed, the seminal principle will still be 

 proved to be a compound very similar to the albuminous 

 substances ; but it is not real albumen, as it does not 

 coagulate by boiling. The seminal fluid contains, more- 

 over, some true albumen combined with soda. Some 

 fat and inorganic substances are also found in it ; but 

 their properties and proportionate quantities have not 

 been accurately investigated. 



18. An albuminous compound, which as such is 

 found in the blood, though in small quantity, is abun- 

 dantly collected towards the end of pregnancy and after 

 confinement, in the breast or lacteal gland of the female. 

 It is the caseine, which, as contained in milk, forms an 

 essential alimentary principle for the infant. 



Caseine is the substance which coagulates at the 

 surface of boiling milk, in the form of folded, wrinkled 

 membranes, which, as often as they are taken off, ara 

 succeeded by others. It is completely and rapidly sepa- 

 rated in thick flakes, if milk, which when fresh is alka- 

 line, is boiled with an acid for instance, with acetic 

 acid. In this way milk is separated into a firm part, 

 which contains, besides easeine, a great quantity of fat ; 

 and into a liquid, in which are sugar of milk, the rest of 

 the milk-fat or butter, certain inorganic substances, and 

 the acid which has been added ; this last liquid represents 

 the well-known whey. 



The sugar of milk is a constituent of fat, like the 

 sugar of grapes, into which the former can easily be 

 transformed by acids. Sugar of milk differs from tho 

 sugar of grapes in being incapable of vinous fermenta- 

 tion by the addition of yeast. This fermentation con- 

 sists in tho change of sugar of grapes into spirit of wino 

 and carbonic acid ; but as acids transform sugar of milk 

 into sugar of grapes, the former also is mediately fer- 

 mentable. Sugar of milk is soluble in water, and 

 consists, like lactic acid, of carbon, hydrogen, and 

 oxygen, in the same proportions as sugar of grapes. 



It is peculiar to sugar of milk, that its sweet taste is 

 very slight. Here also we see, as formerly, when speak- 

 ing of acids and salts, that taste is a very deceptive test 

 of chemical distinctions. As sugar of grapes is found in 

 the chyle and blood, and corresponds in its composi- 

 tion and properties very nearly with sugar of milk, the 

 latter is therefore to be derived from the sugar of grapes, 

 and also from the amylaceous substances of our nutri- 

 ment, which are transformable into sugar. 



When we speak in ordinary life of butter, this Las a 

 much wider signification than chemists attach to the 

 name of butter-fats, or butyrine. In the latter sense, 

 we understand by it a neutral fat fluid at ordinary 

 temperatures which, when combined with alkalies, pro- 

 duces a soap, the acid of which, when free, is very vola- 

 tile, .and possesses a strong acute smell and taste of 

 butter. This acid is, therefore, called butyric; but in 

 butter, butyriae is mixed with oleine and margarine; 

 and by the sapouification of butter, we not only obtain 

 alkali-salts of oleic, margaric, and butyric acids, but also 

 of three other fatty acids, which, like butyric acid, are 

 remarkable for their volatility and pungent smell. We 

 shall call them caseic, sudoric, and capric acids (capronic, 

 caprylic, and caprinic acids of other authors) 



While butyric acid, with other fatty acids found in 

 our bodies compounded with glycerine or alkalies, con- 

 tains, in proportion to oxygen, the least quantity of 

 carbon and hydrogen, the two latter elements are more 

 abundantly represented in the three other volatile acids 

 of the butter namely, caseic, sudoric, and capric acids; 

 the first containing the smaller quantities. 



