IIV.XUER AND THIKST.] 



CHEMISTRY. 



337 



of phosphates of lime and iron, are also dissolved in the 

 perspiration. 



To the perspiration may bo added the tears. These 

 are continually secreted in a very small quantity by a 

 gland situated in the external corner of the eye, under- 

 neath the skin. From hence they flow over the surface 

 of the eyeball to the internal corner, where they are re- 

 absorbed by an aperture in each eyelid, leading by a 

 minute canal into the nasal cavity, to be excreted with 

 the nasal mucus. 



The tears are a very diluted solution of common salt, 

 mixed with some detached cellules from the external 

 coating of the eyebalL 



The sebaceous matter of the skin is also a mixture of 

 detached epidermic cellules with some other substances, 

 of which fat and some salts are the principal components. 

 It has, however, a diiferent composition in different 

 parts of the skin. The ear-wax, for example, is remark- 

 able for a bitter yellow substance, soluble in spirits of 

 wine, and for a portion of gall-fat. 



30. External Excretions. Just as a part of the in- 

 testinal mucus is voided as an excrementitious matter, 

 so the mucus of the other parts of our body of the nose, 

 windpipe, lungs, urinary ducts, and the several organs 

 principally of women is to be considered an excremen- 

 titious substance. 



As a considerable part of the mucus is represented by 

 horny substances which coat the mucous membranes, with 

 these excretions may be included those parts of the hair 

 ;i:id nails which, being cut off and growing again as if 

 the younger parts pushed the older parts upwards form 

 a regular part of the expenditure of the body. It is the 

 same with the epidermis, which is continually scaling off. 



The organic origin of all these homy substances is to 

 be derived from the albuminous compounds, which are, 

 by their transformation into the former, separated from 

 the blood ; one part of a tissue is thus directly lost with- 

 out having passed again through the blood from which it 

 originated. 



ON HUNGER AND THIRST. 



31. Principle of Reparation. It has been mentioned, 

 in a previous section, that one-third in weight of the food 

 we take during twenty-four hours is lost with the urine, 

 and another third with the air exhaled. Another third 

 i >f the consumed aliments daily leaves our bodies in the 

 form of excrements, perspiration, mucus, fat, tears, and 

 detached horny matter. 



It is evident that this does not mean an immediate 

 excretion of alimentary principles as such ; for, except- 

 ing the undissolved remains of food which the rectum 

 expels with the excrements, there is no substance in our 

 excretions which had not passed from the mouth through 

 the blood to the tissues, and from these back again through 

 the blood to the organs of excretion. Thus, for ex- 

 ample, the carbonic acid and water which we exhale, have 

 once been fat or albumen, and the urea has nourished 

 the tissues in the form of an albuminous constituent of 

 the blood, before the kidneys convey it as a useless sedi- 

 ment of the body to the expolling bladder. 



This main result is not affected by the indirect mode 

 of the excretion of our nutriment. The weight of a 

 healthy grown-up person does not undergo any sensible 

 alteration from one day to another ; for as much as is 

 subtracted from the body by excretion, is conveyed to it 

 ii;:iiii by the food which is digested. 



Thus experience teaches, when the total amount of 

 excretion diminishes, the weight of food we take de- 

 creases also. If the proposition could also be transposed 

 if it were true that, with a diminished quantity of food, 

 a proportionate diminution of the secretions succeeded, 

 then we could learn the art of fasting, and the horse of 

 the well-known tale would not have died the day before 

 he promised to crown the hopes of his parsimonious 

 master with the most brilliant realisation. 



It is not, however, thus. Even when abstaiiu'ng from 

 all solid and liquid food, we exhale carbonic acid and 

 water ; the excretion of urine and excrements continues 



VOL. I. 



all the same, the hair and nails grow, and perspiration 

 and mucus hourly subtract from the body its most 

 essential constituents 5 and this abstinence, if continued, 

 is only too soon followed by considerable diminution in 

 the weight of our body. 



Food only is able to obviate this decrease of weight ; 

 and as excretion takes place even if we take no food, it 

 is less correct to say that we excrete again the aliments 

 we have taken, than that the food restores what has been 

 lost by the excretions. For carbonic acid, urea, salts, 

 water ; we take in exchange amylaceous and fatty 

 matters, albumen, and inorganic substances. On this 

 process of exchange, the metamorphosis of tissues hinges ; 

 the alimentary principles are, therefore, very often, and 

 quite correctly, denominated the matters of reparation. 



32. Effects of Inadequate Aliment. If the supply 

 cease while the expenditure continues, the composition 

 of the tissue is immediately changed ; and the blood, 

 which receives aliment not only for the tissues, but also 

 for itself, fails in a few days, or at the longest, in a few 

 weeks for the oxygen we inhale wastes the blood when 

 supplies are stopped. The constituents of the body con- 

 tinue, as before, to succumb to the influence of that 

 mighty agent in the decomposition of organic matters. 



The changes in composition produced in the blood and 

 the tissues by a deficient supply, can only be distinctly 

 observed after some tune. Then we first of all notice the 

 fat wasting away a proof that the fats are more 

 accessible to the action of oxygen than the albuminous 

 substances. Carbon and hydrogen are the elements 

 which combine with oxygen the most readily ; and upon 

 this depends their remarkable combustibility. The 

 emaciation is, therefore, explained by the fact, that the 

 fat substances surpass the albuminous in their quantity 

 of carbon and hydrogen. 



Next to the fats, those organs lose the most rapidly 

 in weight which are remarkable for their great amount 

 of albuminous substances. The muscles, the heart, the 

 spleen, and the liver thus waste away. 



From this speedy decomposition is excepted only one 

 part of the human body, and that the part of which we 

 should be least of all disposed to predicate a slow process 

 of change ; for, although the brain and the nerves con- 

 sist almost exclusively of fat and albumen, two of the 

 most mutable substances of our body, we know from 

 investigation in cases of animals which have died of 

 starvation, and of men who have succumbed to pro- 

 tracted diseases, that these organs have suffered the 

 least loss of weight. The reason of this is an unresolved 

 enigma. There is but one probable explanation for this 

 strange circumstance in the peculiar manner in which 

 albumen and fat are combined in the brain and the 

 nerves. If this combination, however, and its resistance 

 to the influence of oxygen, cannot be sufficiently ex- 

 plained, we still find in this fact the best explanation of 

 the late decay of the mental faculty, which we see so 

 often flame up again with fallacious vivacity in the last 

 moments of expiring life. 



Slower than the fat and the muscles, but faster than 

 the brain and the nerves, the bones, the cartilages, the 

 skin, and the lungs waste away ; including, in a word, 

 all parts composed of gelatine, horn, and elastic fibres. 

 These tissues owe their greater tenacity of existence to 

 the difficulty with which they dissolve, which gives them 

 greater power to resist the action of oxygen ; for though 

 the assertion of former chemists that only dissolved 

 bodies act upon each other cannot be admitted without 

 exceptions, yet there is scarcely any condition more 

 favourable to chemical combination and decomposition 

 than a state of solution. 



33. Effects of Abstinence from Food. From a con- 

 sideration of the intimate reciprocal action by which the 

 blood is combined with the tissues, secretions, and ex- 

 cretions, the fact that, with a failing supply, the com- 

 position of the tissues still changes and the excretions 

 continue, necessarily indicates an altered composition 

 and diminished quantity of the blood. But while 

 science endeavours to find the laws upon which this 

 alteration in the composition of the blood is based, it 



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