140 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. 



and is rendered as dark as venous blood ; or even darker, if exposed 

 very completely to its influence. The simple removal of this carbonic 

 acid is not sufficient to restore the original colour ; for this removal may 

 be effected by hydrogen, which has the power of dissolving out (so to 

 speak) the carbonic acid diffused through the blood ; but the arterial 

 hue is not restored unless oxygen be present, or saline matter be added 

 to the blood. Recent observations seem to render it probable that 

 these variations are due, not so much to changes of composition in the 

 Hasmatine, as to changes of form in the Corpuscles which contain it. 

 For when the haematine has been separated and diffused through water, 

 it is neither darkened by carbonic acid, nor brightened by oxygen, 

 unless some corpuscles be floating in the solution. And it appears 

 that the effect of oxygen, like that of saline solutions, is to contract the 

 corpuscles and to thicken their walls, thus, by altering their mode of 

 reflecting light, making them appear bright red ; whilst carbonic acid, 

 like water, may be seen to occasion a dilatation of the corpuscle, and 

 a thinning of its walls (which are at last dissolved by it), in a degree 

 that is probably sufficient to account for the darkening of the hue of 

 the mass. 



223. These changes in the condition of the Red corpuscles (whatever 

 their precise nature may be), taken in connexion with the fact, that 

 these bodies are almost completely restricted to the blood of Vertebrata 

 (whose respiration is much more energetic than that of any Invertebrated 

 animals save Insects, which have a special provision of a different cha- 

 racter), and that their proportion to the whole mass of the blood corre- 

 sponds with the activity of the respiratory function, leave little doubt 

 that they are actively (but not exclusively) concerned as carriers of 

 Oxygen from the lungs to the tissues, and of Carbonic acid from the 

 tissues to the lungs ; and that they have little other direct concern in 

 the functions of Nutrition, than the fulfilment of this duty. Their com- 

 plete absence in the lower Invertebrated animals, in the earliest condi- 

 tion of the higher, and in newly-forming parts until these are penetrated 

 by blood-vessels, seems to indicate that they have no immediate con- 

 nexion with even the most energetic operations of growth and develop- 

 ment ; whilst, on the other hand, there is abundant evidence, that the 

 normal activity of the animal functions is mainly dependent upon their 

 presence in the blood in due proportion. 



224r Next in independence to the cells or corpuscles floating in the 

 animal fluids, are those which cover the free membranous surfaces of 

 the body, and form the Epidermis and Epithelium. Between these 

 two structures there is no more real difference than there is between 

 the Skin and the Mucous membranes. The one is continuous with the 

 other ; they are both formed of the same elements ; they are cast off 

 and renewed in the same manner ; the history of the life of the indivi- 

 dual cells of each is nearly identical ; but there is an important diffe- 

 rence in the purposes which they respectively serve in the general eco- 

 nomy. The Epidermis or Cuticle covers the exterior surfaces of the 

 body, as a thin semitransparent pellicle, which is apparently homoge- 

 neous in its texture, is not traversed by vessels or nerves, and was 

 formerly supposed to be an inorganic exudation from the surface of the 



