EXCRETIVE FUNCTIONS.] 



CHEMISTRY. 



335 



rather conjectured than proved, has been triumphantly 

 demonstrated by more recent investigations ; and it is 

 now known that the constituents of the tissues gra- 

 dually deposit a sediment, which, being unfit for the 

 functions of the organs in which it accumulates, is con- 

 veyed back to the blood, in order to be excreted from it. 



The most important substances which are excreted, 

 have, in the course of physiological science, been traced 

 from the blood into the tissues. The blood contains 

 urea; the vitreous humour of the eye does also ; carbonic 

 acid is present in a large portion in the blood, and is 

 likewise to be found in every solid part of our bodies ; 

 and carbonic acid and urea are the principal products of 

 decomposition, which the lungs and kidneys abstract 

 from the blood, in order to remove them as waste mate- 

 rial from the body.* 



A process which ceases not even for a second, and in 

 which one development is continually following another 

 without any cessation, can evidently be marked at cer- 

 tain stages of the transformation only. Therefore we 

 cannot make out, in an uninterrupted chain, the whole 

 series of intermediate conditions through which the albu- 

 minous and adipose substances pass, before being wholly 

 decomposed into those simple products, urea, carbonic 

 acid, and water. But since, through the increasing 

 number of investigators, aided by instruments of 

 greater delicacy, more and more points of transitiot 

 been found in the series conducting from the con- 

 ' 'f the blood, through the tissues, to the excre- 

 tion*, we are able confidently to assert, that we are 

 pursuing our investigations in the right direction. 



\Ve cannot, at present, draw any distinct limit be- 

 tween substances belonging to the development, and 

 those belonging to the tissues. JJut no greater praise 

 can be bestowed upon the physiologist, than that he 

 breaks through the barrier by wliich it has too often 

 1 to enclose certain parts of nature, and 

 to blockade the understanding of man. The transitions 

 of nature are numberless. The boundary-lines between 

 the several classes of natural bodies lose their distinct- 

 in proportion to the extent and depth of our 

 scientific kn 



To the substances transitional between development 



and regressive transformation, belongs a combination 



4ing of nitrogen, carl ton, hydr. >_,'!), and oxygen, 



which is found in the juice of flesh, and wliich may be 



I the flesh-principle, or krcatine. it is not yet known 



what intermediate substances connect this principle with 



the albuminous compounds from which it origii. 



ily know, that by thy mere presence of acids, which 

 are never absent from the flesh, kroatino is transformed 

 into an alkaline substan , which is to be found, not 

 only in the flesh, but also in the urine. This substance 

 iniy bo called the flesh-basis, or krcatinine, and con- 

 sidered as decidedly an excremontitious body, which has 

 passed into that stage in the tissues themselves ; being 

 formed from the albuminous matters, and having passed 

 beyond the intermediate state of the kroatine. The 

 peculiar to the flesh, technically called iuosinic : 

 wliich is to bo found in the muscles, associated with 

 lactic acid, most probably belongs also to the products 

 of regressive transformation, although it has not hit! 



dJMOTWad in any excretion, but only in the flesh 

 itself. 



Like the flesh-principle, so are the peculiar basis and 

 acid of the flesh, compounded of nitrogen, carbon, 

 hydrogen, and oxygen. The flesh-principle is trans- 

 fiirmud into flush-basis by the subtraction of water only. 

 Flesh-acid is remarkable for containing a high propor- 

 tion of oxygen. While the flesh-principle, as such, com- 

 bines neither with bases n >r acids, the llesh-basis unites, 

 as its name indicates, with acids, and the llt-sh-acid with 

 alkalies. Flush-basis and flesh-acid an: e.nily dissolved in 

 cold water ; the flesh-principle with more < lilneulty, requir- 

 ing boiling-water, which dissolves it in great abundance, 

 illustrative example* may be suttiuient to es- 

 tablish the general proposition, that the formation of 

 the excremeutitious bodies begins in the tissues them- 

 Seo ante, p. 326. 



selves, and that the function of the excretive glands is 

 mainly to attract and elaborate these substances from 

 the body ; for the products of the regressive metamor- 

 phosis pass from the tissues into the blood, and from 

 the blood into the excretive organs. 



26. Excreting Glands. If among these glands the 

 most important place is ascribed to the respiratory 

 organs, the conviction is founded upon the fact, that 

 the functions of the lungs can least of all bear inter- 

 ruption without disturbing, or altogether destroying, 

 the activities essential to life. That element in the 

 surrounding atmosphere, therefore, which is absolutely 

 necessary for respiration, may be called vital air. This 

 vital air is the oxygen, which is the mightiest agent in 

 producing the metamorphosis of tissues. 



The air we breathe is a combination of oxygen with a 

 large proportion of nitrogen, a little aqueous vapour, 

 and still less carbonic acid. While the oxygen at every 

 inspiration enters the lungs in order to impart to the 

 blood its proper composition, the carbonic acid of the 

 atmosphere, on the other hand, originates from the 

 breath given out at every expiration from the lungs of 

 men and animals. Valuable researches have shown that 

 this carbonic acid penetrates from the air into the- leaves 

 of plants, in order to supply the principal nutriment to 

 these numberless green and flowering products of nature, 

 which form such an essential condition of animal life : 

 not as nutriment alone, for it is the vegetation of the 

 earth wliich gradually decomposes the carbonic acid 

 which it has absorbed, and whilst it keeps the carbon 

 for the formation of its own tissues, exhales the greater 

 part of the oxygen ; the latter being called, as we have 

 seen, and with the greatest propriety, tho vital air of 

 men and animals, t 



The carbonic acid which wo breathe out is, as before 

 mentioned, to be traced so far back as the tissues them- 

 selves; from these it penetrates into the capillaries in 

 order to pass into certain canals, in which the blood 

 flows in a direction opposite to that of the arteries. 

 These canals are called reins, and are distinguished from 

 the latter by not possessing pulsation. 



The blood which the arteries have conveyed from tho 

 heart to the remotest organs of our body, having been 

 changed by the substances which it has lost, and by 

 others wliich it has taken up during its course, flows 

 through the veins back again to the heart, thus forming 

 a true circulation. This circle is completed by tho 

 capillaries, which connect the arteries with the veins. 



Tho venous blood contains an abundant quantity of 

 carbonic acid ; but this product of decomposition does 

 not originate exclusively from the tissues, and pass 

 therefrom into the veins, inasmuch as the arterial blood 

 itself is already impregnated with a certain quantity of 

 carbonic acid, formed in this part of the circulation. 

 The venous blood, however, contains more carbonic acid 

 than the arterial, the former receiving an additional 

 supply of this gas from the tissues. 



Besides the great circulation from the heart, through 

 the tissues, and back again to the heart, there exists 

 also a small circulation of the blood ; for all tho blood 

 of the veins, which flows into a special compartment of 

 tho heart, is propelled therefrom into tho lungs, and 

 returns from these into the heart, after having been 

 changed in a peculiar way. The blood flowing from the 

 heart to the lungs is dark-red, even brown-red, contain- 

 ing but little oxygen and much carbonic acid ; and this 

 is the character of the venous blood in general. But in 

 {lie lungs, to which an abundant quantity of oxygen ia 

 conveyed with the inhaled air, carbonic acid and aqueous 

 vapour pass from the venous blood into this organ, and 

 are compensated by the inhaled oxygon which transudes 

 into these vessels. By this process the blood becomes 

 light-red, containing less carbonic acid and water than 

 that of the veins. The blood thus transformed, returning 

 from the lungs, is now called arterial, as it corresponds 

 with that of the arteries. It is the blood of the arteries 

 themselves, because it is conveyed to them by the heart, in 

 order to permeate all tissues again with nutritious juice, 

 t See ante, p. 315. 



