104 CIRCULATION OF THE BLOOD IN THE VESSELS. 



The blood' in its passage through the lungs does not meet with the resist- 

 ance which is presented in the systemic circulation ; and the anatomy of the 

 pulmonary vessels and of the right side of the heart shows that the blood 

 must circulate in the lungs with comparative facility. The power of the 

 right ventricle is evidently less than half that of the left, and the pulmonary 

 artery will sustain a much less pressure than the aorta. 



The two sides of the heart act simultaneously ; and at the same time that 

 the blood is sent by the left ventricle to the system it is sent by the right 

 ventricle to the lungs. The pressure of blood in the pulmonary artery, meas- 

 ured by connecting a cardiometer with a trocar introduced into the pul- 

 monary artery of a living horse through one of the intercostal spaces, has 

 been found to be about one-third as great as the pressure in the aorta, 

 which corresponds pretty nearly with an estimate of the comparative power 

 of the two ventricles, judging from the thickness of their muscular walls 

 (Chauveau and Faivre). 



On microscopical examination of the circulation in the lower animals, as 

 the frog, the movement of blood in the capillaries of the lungs does not pre- 

 sent any differences from the capillary circulation in other parts, except that 

 the vessels seem more crowded with corpuscles and there is no " still layer " 

 next their walls. 



Circulation in the Walls of the Heart. The circulation in the walls of 

 the heart does not present any important peculiarities. It has been shown 

 that the pressure of blood in the coronary arteries in the dog, during the 

 ventricular systole, is sufficient to supply the arterioles in the substance of 

 the heart with blood precisely as it is supplied to the general arterial system. 

 In a number of experiments, in which simultaneous traces of the pulse-beats 

 were obtained, it has been found that the coronary and carotid pulses were 

 practically synchronous (Martin). 



Passage of the Blood- Corpuscles through the Walls of the Vessels (Diape- 

 desis). In the frog it has been observed that the leucocytes sometimes pass 

 through the walls of the blood-vessels, either by means of small orifices 

 (stromata) or by a kind of filtration through the substance which unites the 

 borders of the endothelial cells. This phenomenon was described by Waller, 

 in 1841, but has attracted much attention since the more recent researches 

 of Cohnheim. In this process it is observed that the leucocytes, which first 

 adhere to the vascular walls, send out little projections which penetrate the 

 membrane, so that a point appears on the outside of the vessel. This point 

 becomes larger and larger, until 'the entire mass of the corpuscle has passed 

 through. The corpuscles then migrate a certain distance by means of the 

 movements known as amoeboid, which have already been described. It was 

 supposed by Cohnheim that this was one of the early phenomena of inflam- 

 mation, the migrating corpuscles afterward multiplying by division, consti- 

 tuting the so-called pus-corpuscles. Following stasis of blood in the small 

 vessels, the red corpuscles, it is supposed, pass out in the same way. It is 

 not certain that diapedesis, even of leucocytes, is a normal process or that it 

 takes place in the human subject. According to Hering, the red corpuscles 



