Cause of the change of colour in the blood. — The manner in which the changes of colour in the blood is effected as it passes through the pulmonie and systemic capillary vessels, has not yet been satisfactorily determined. It seems now t,o be pretty generally admitted that the hmrnatosine or colouring matter of the blood is enclosed within the enveloping membrane of the red corpuscles ; that this hmmatosine, though it may be combined with iron, does not derive its colour from the pre sence of this metal ; and that all attempts to explain the change in the colour of the blood in the lungs by the formation of certain oxides and salts of iron must be abandoned. It is well known that various substances, besides oxygen gas, can impart a bright red colour to venous blood when mixed with it, and without being attended with any evo lution of carbonic acid gas. The best known of these are solutions of' the sulphate of soda, nitrate of potass, phosphate of soda, carbon ate of soda, carbonate of potass, and sugar.
The opinion of Stevens*, that the change from the venous to the arterial hue in the blood is to be attributed to the actions of the salts dissolved in the blood upon the hmato sine, after the removal of the free carbonic acid of the venous blood through the attrac tive force of the oxygen of the atmospheric air, has not been confirmed by subsequent researches. It has been ascertained that the removal of carbonic acid from venous blood, by means of the air-pump -I-, or by agitation with hydrogen gas * and the addition of a saline solution, of the same strength as that existing in the blood t, will not impart to it the arterial hue, if oxygen gas be not at the same time present. The oxygen gas, there fore, acts directly, and not indirectly by re moving the carbonic acid, in changing the colour of the blood; but as a small quantity only of this gas is sufficient, when the salts are present in their usual quantity, to produce this effect I, the action of the oxygen, in changing the colour of the blood in respira tion, will be aided by the presence of the salts.
In the present state of our knowledge, there is some difficulty in deciding whether the reddening of the blood by the absorbed oxygen be entirely a physical action, or whether it be partly physical and partly che mical, seeing that several accurate observers, who have recently investigated this point, have arrived at very different conclusions.
The opinion, first promulgated by Dr. Wells§, that the change from the venous to the arterial hue arises from an increased re flection of light in the red particles, caused by the presence of the absorbed oxygen, and without any, chemical change upon the hwma tosine, has of late obtained several supporters. Those who have adopted this view do not, however, agree in their explanation of the manner in which this increased reflection of light is effected; some maintaining that it arises from an alteration in the form of the red corpuscles, and that this change consists in the biconvex corpuscles of the venous blood, becoming biconcave in the arterial blood II ; while others believe that the action of the oxygen on the blood is analogous to that of the nitrous oxide on the solutions of the salts of iron, changing their colour out entering into chemical union with them.* We may, in the meantime, conclude that the change in the blood from the venous to the arterial hue in the lungs, is a physical and not a chemical action ; and that though there is pretty strong evidence in favour of the opinion that this physical change consists in an alteration of the form of the red corpuscles, yet it is not free from doubt.
gnawing. The molar teeth have their crowns flattened and traversed by plates of enamel, arranged transversely', the better to antagonise the backward and forward movement of the jaws.