These two sorts of impediment coincide with each other, in many instances. Thus in asphyxia, a part of the black blood which circulates in the arteries stops in the face, upon the mucous surfaces, in the tongue, and in the lips, while the other, and much the larger quantity, finds no impediment in the general capillary system, and is finally arrested in the lungs. What is the reason, why certain parts of the capillary system refuse to admit the venous blood, or if they admit it, do not pass it on to the veins; while others are less enfeebled by it, and transmit it as freely as ever. All this must certainly depend on the relation existing between the sensibility of each part and the venous blood. I was desirous of making use of the power, which we possess, of changing the colour of the blood, for getting some insight into the influence of the circulation of the mother, upon that of the fœtus; accordingly I procured a bitch big with young, and asphyxiated her, by closing a tube, adapted to the trachea. About four minutes after she had ceased to breathe, I opened her; the circulation was going on. I then cut into the matrix, and exposed the cord of two or three of the fœtuses. The artery and the vein, were both of them full alike of venous blood. Had I been able to procure other bitches in a similar state, I should have repeated this experiment in another manner. I should in the first place have compared the natural colour of the vein, with that of the artery. In many of the young of the guinea pig, the difference appeared to me to be much less than it is in the adult animal. In many circumstances indeed I could perceive no difference whatever. Both the arterial and venous blood were equally black, though the respiration of the mother was in no wise impeded by the opening of the belly. Secondly, I should have closed the tube in the trachea, and then have observed whether the change in colour of the umbilical artery of the fœtus (supposing the blood of the artery to be different from that of the vein) were correspondent with that, which would inevitably take place in the blood of the mother. Experiments made with a view to these circumstances, and on large animals, might probably throw much light upon the mode of communication, between the mother and the fœtus. Observations are also much to be desired, with respect to the colour of the blood in the human fœtus, and the cause of its passage from a livid colour, to the very marked red which it assumes, some little time after birth.[95] I might add a number of examples to these, which I have already related of the blackening of the organs by the venous blood. Thus, the kidney of a dog exposed, while the animal is dying of asphyxia, is much more livid than in its natural state, the spleen also and the liver, when divided, emit only black blood, instead of that mixture of red and black blood which is observable, in the section of these organs, upon an animal which breathes freely.