BLOOD-DISCS OF BIRDS AND REPTILES.

205

seen in the corpuscles of circulating or of freshly-drawn blood, as it is in that of blood which has been drawn for some little time; and it is best brought into view by treating the blood either with water or with acetic acid. The long diameter of the oval discs of BIRDS (fig. 116) varies from about 1-1700th to 1-2400th of an inch; and the short diameter from about

At A are seen the red corpuscles a, b, and the colourless corpuscle c; at B, a red corpuscle treated with acetic acid.

1-300th to 1-4800th. Thus the discs, though much longer than those of Man, are not in general much broader. In REPTILES,

Fig. 118.-BLOOD CORPUSCLES OF PROTEUS.

a, b, red corpuscles; a*, corpuscle showing the nucleus; c, colourless corpuscle; d, red corpuscle treated with water.

206

BLOOD-DISCS OF REPTILES AND FISHES.

there is considerable diversity as to the size of the discs; but the largest particles are found in the group of Amphibia, and especially in those species which retain their gills through life. The oval discs of Frogs (fig. 117) have a long diameter of about 1-1000th of an inch, and a transverse diameter of about 1-1800th. Those of the perennibranchiate Amphibia (§ 87) may even be distinguished by the naked eye; those of the Siren having a long diameter of about 1-435th of an inch, whilst in the Proteus (fig. 118) the long diameter is stated occasionally to reach 1-337th of an inch. In FISHES, also, the size of the

blood-discs is variable; they are sometimes smaller (fig. 119), though generally larger, than those of the Frog; but they never approach those of the last-named remarkable animals. Hence the great size of the blood-discs of the curious Lepidosiren (fig. 41) is strongly indicative a, a, b, red corpuscles; c, colour of the Reptilian affinities of that

Fig. 119.-BLOOD CORPUSCLES OF

ROACH.

less corpuscle; d, red corpuscle treated with water.

species.

231. It is by observing the large blood-discs of the Frog, and still better those of the Proteus and Siren, that we can obtain the best information as to their structure. They are evidently flattened cells, having an envelope or cell-wall, which consists of an extremely delicate membrane, and which con tains a fluid. The nucleus consists of an assemblage of minute granules, which seem adherent to each other and to the wall of the cell; and it corresponds, in all essential particulars, to the nuclei of the cells of other Animal tissues (§ 32). The fluid contained in the cells has a red colour; and it is to this that the peculiar hue of the blood of Vertebrata is owing. When we are looking at a single layer of blood-discs, however, their red colour is not apparent, but they have rather a yellowish tint; and it is only when we look through a number at once, that the characteristic hue is seen. The fluid is of about the same density as that in which the particles float; and thus neither will have a tendency to pass towards the other. But, if we dilute the liquor sanguinis with water, the fluid outside the cells will have a tendency to pass towards their interior, according to the law of Endos mose. The cells will in consequence be first distended, and

STRUCTURE AND COMPOSITION OF RED CORPUSCLES. 207

will then burst; and their contents will be diffused through the surrounding fluid, whilst their membranous walls will subside to the bottom. On the other hand, if the liquor sanguinis be rendered denser than the fluid in the blooddiscs, as by the admixture of gum or syrup, the latter will pass towards it, and the cells will become still more flattened, and more or less completely emptied. The flexibility and elasticity of the blood-discs are well seen, in watching (with a microscope) its flow through the minute vessels; for if one of them meets with an accidental obstruction to its progress, its form becomes accommodated to that of the space left for it to pass, and it makes its way through a very small aperture, recovering its usual form immediately afterwards.

232. The Red Corpuscles differ considerably in chemical composition from the liquid in which they float. Of the solid residue obtained by drying, about one-eighth is formed by their cell-walls, the remainder being yielded by the cellcontents. The latter portion seems to consist chiefly of a mixture of two components, which have been named globulin and hæmatin. The former is a colourless substance, nearly allied to albumen in composition, but differing from it in some of its reactions; its most characteristic peculiarity, however, being its power of crystallizing. Its crystals, the form of which varies in different animals, are usually tinged deeply with hæmatin, from which they cannot easily be freed. The composition of hæmatin, to which alone the colour of the red corpuscles (and consequently of the whole mass of the blood) is due, is notably different from that of the albuminoid compounds; the proportion of carbon to the other components being much greater, and a definite quantity of iron being an essential part of it. This iron, in a certain state of oxidation, has been supposed to be the source of the red colour; but such is certainly not the case; and this hue must be, like the colours of Plants, a peculiar attribute of the organic compound which presents it.-Besides their globulin and hæmatin, the red corpuscles contain a certain proportion of fatty and mineral matters. The former, which are united with phosphorus, are of a kind which are scarcely traceable in the liquor sanguinis; and the latter are remarkable as having potass for their principal base, whilst the base of the salts of the liquor sanguinis is chiefly soda. Hence it appears that

208

PROPORTION OF RED CORPUSCLES.

the Red Corpuscles draw into themselves nearly the whole of the iron, phosphorus, and potass, which the chyle pours into the circulating current; and that they modify a large proportion of the solid matter of the blood, that which they contain being notably different in composition from that of the liquor sanguinis, which does not differ, save in the proportion of its components, from the liquid portion of Chyle or Lymph. 233. The proportion of Red Corpuscles to the whole mass of the blood varies greatly in different animals, and even in different states of the same animal. It is greatest in those which have the highest muscular vigour and activity, and which consume the largest quantity of oxygen by respiration; hence these particles are rather more numerous in the blood of Birds than in that of Mammals, and far more abundant in these last than in Reptiles or Fishes. Again, they are more numerous in Men of ruddy complexion, strong pulse, and active habits, than in those of pale skins, languid circulation, and comparatively feeble powers. In a healthy Man they seem to constitute about half the mass of the circulating blood; but they contain as much as three-fourths of its solid matter, the proportion of dry corpuscles being about 150 in 1000 parts of blood, whilst that of the other solid matters is about 50. A very marked decrease occasionally presents itself in disease; the proportion of dry corpuscles being sometimes reduced as low as 27. When too abundant, they produce what is known as the plethoric condition of the body, in which hæmorrhage from the bursting of a blood-vessel is liable to occur. Their number is effectually reduced by bleeding; and the aspect of those who have suffered from extreme loss of blood, gives sufficient evidence that the deficiency is not made-up for a long period. The most effectual means of restoration, in cases where the proportion of blood-corpuscles is too low, is a highly nutritious diet, with the administration of iron as a medicine; for this substance seems to have the power of hastening the reproduction of the corpuscles, being itself an essential ingredient in their contents; and there are facts which show its remarkable power of increasing their amount in proportion to the mass of the blood.

234. It appears that the red corpuscles, like other cells, have a certain allotted term of life; and as they are continually dying, they must be as continually reproduced. The

COLOURLESS CORPUSCLES-USES OF RED CORPUSCLES. 209

mode in which this reproduction is effected has not yet been clearly made out; but there is strong reason to believe that the red corpuscles are developed from the corpuscles of the chyle and lymph (§ 222) which are continually being poured into the circulating current, and of which isolated examples, known as the white or colourless corpuscles, are met with in every drop of blood that is examined under the microscope. The size of these is pretty much the same in all Vertebrata, their diameter being usually about 1-3000th of an inch. In the blood of Man and the Mammalia in general (fig. 115, D) they are not easily distinguished from the red particles; their diameter being nearly the same, while the colour of single discs of the two kinds is not very dissimilar. But in the lower Vertebrata, whose blood has large oval red particles, the difference between the two kinds is very obvious; and the resemblance which the colourless globules (c, figs. 116-119) bear to those of the chyle and lymph, is very striking. Similar colourless particles exist, to a variable amount, in the nutritive fluid of Invertebrated animais; so that in this, as in some other respects, that fluid bears a stronger resemblance to the chyle and lymph of the Vertebrata, than it does to their blood, which is characterised by the presence of the red particles.

235. Physiologists are now generally agreed, that one of the functions of the Red Corpuscles is to convey oxygen from the lungs to the tissues and organs through which the blood circulates, and to bring back the carbonic acid which is set free in these, so as to deliver it at the lungs. For although it is certain that the liquor sanguinis can also convey these gases, yet experiment shows that the red corpuscles can take up, bulk for bulk, a much larger proportion of them; and that the blood which is richest in these particles is, therefore, most fit to serve as the medium for the transmission between the respiratory organs and the body at large. Now it is in the nervo-muscular apparatus that there is the greatest demand for oxygen; for this apparatus is not capable of vigorous action, unless oxygen be freely supplied to it. The quantity of this it requires, however, depends upon the exercise of its powers; for when at rest, it needs little or no more than is made use of by the other tissues; but whilst in activity, it needs a greatly-increased supply. The quantity of oxygen which the animal takes-in by its lungs, and the amount of

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