[82] The observation of Bichat is very just; and I have myself often observed in cases of apoplexy, that the motions of the heart continue many hours after the arteries contain only black blood. [83] At the period Bichat wrote, it was impossible to know whether the arterial or venous blood contained most hydrogen and carbon. At the present day even, when the means of analysis are much more perfect and animal chemistry farther advanced, we are hardly better informed. [84] It seems that when Bichat wrote this work, he had not fixed in his own mind what part the arteries take in the circulation; at least, in this paragraph, he seems to give them a contractile power, which, in his other works, he accords only, to their ultimate ramifications. [85] It is certain, whatever Bichat may say concerning it, that numerous angles in the course of a blood-vessel, by increasing the friction, must retard the course of the fluid which runs through its cavity. We may in fact believe that it is one of the causes, which, in the flattening of the lungs, embarrass the circulation; this cause exerts its action principally upon the last ramifications of the pulmonary artery which, by their interlacing, form the bronchial cells; another more powerful cause, which acts equally on the divisions of the vein and the artery is, as we have said, the pressure made on the lungs by the flattening of the thorax, and the elevation of the diaphragm. [86] See the article on the influence of the lungs on all the parts. I am compelled here to deduce consequences from principles which I shall prove hereafter; such is in fact the connexion of questions which have the circulation for their object, that the solution of one draws as a necessary consequence that of all the others. It is a circle in which it is always necessary to suppose something, leaving it to be proved afterwards. [87] I cannot see what can have induced Bichat to admit this erethism of the lungs, the use of which cannot be imagined. It is quite enough to suppose in the organs the existence of those concealed properties, when there is need of them to explain their functions. [88] As the blood flows in the veins in a continuous motion, the distension of these vessels does not take place in the direct motion of the blood, but in its retrograde motion. Every time the right auricle contracts, a portion of blood, instead of passing into the ventricle of the same side, is forced back into the superior and inferior venæ cavæ, and into the principal venous trunks which open into them. By this reflux of the blood, these veins are dilated, and have a pulsation which is easily seen in the jugular in very thin people. This pulsation has received the name of the _venous pulse_. When it is very evident, it may indicate an obstacle to the passage of the blood from the auricle to the right ventricle.