THE BLOOD. 113 



It appears, therefore, in the first place, that the* difference 

 in colour of dark and scarlet blood depends, at least partly, 

 on a chemical change in the coloured contents of the cor- 

 puscles; and this agrees with the results of spectral analysis, 

 by which it is proved that the colouring matter, or cruorin 

 as it is sometimes called, if arterial blood, is a different 

 chemical substance from that of venous blood (Stokes). 

 In the second place, it appears that the hemoglobin is 

 the oxygen-carrier in the blood. Indeed, it is proved by 

 direct experiment that serum has little more power of ab- 

 sorbing oxygen than water has. "With regard to the carbonic 

 acid of the blood, although no doubt a large portion of it is 

 known to be contained in the serum, it seems probable, from 

 the effect of that gas on the colouring matter, that the por- 

 tion which is removed in respiration belongs to the cor- 

 puscles. 



83. Before leaving this subject, it may be well to notice an 

 exception to the general rule, that blood returning from the 

 textures is dark not only is that sent to the heart from the 

 lungs scarlet, but the blood returning from certain glands in 

 action is of the same tint. Thus, in experiments on dogs it 

 has been found, that while the blood in the veins coming 

 from the sub-maxillary gland is dark when the gland is at 

 rest, if the nerve (chorda tympani) which supplies the secreting 

 structure be excited, and the gland thus irritated to secrete 

 saliva, a much larger quantity of blood passes through the 

 gland, and it escapes from it scarlet. The blood returning from 

 the kidneys is also scarlet as long as urine is secreted, but 

 is dark when, from any cause, the secretion ceases. In both 

 these instances, it will be observed, that an enormously larger 

 amount of blood passes through the organ than is required 

 for the nourishment of its textures. In the same way, if 

 the blood-vessels of a rabbit's head are paralysed by dividing 

 the sympathetic nerve in the neck, in consequence of the 

 great increase of blood allowed into the part, a portion re- 

 turns unaltered, and the blood is found red in the jugular 

 vein. 



H H 



