160 



penis and of the clitoris, of the bulb and spongy part of the urethra*,, the 

 retiform plexus, which surrounds the orifice of the vagina, and, perhaps, 

 also, the tissue of the spleen, though the experiments of anatomists (Mas- 

 cagni and Lobstein) seem to prove, that in these organs, the arteries and 

 veins, are immediately continuous!. 



LXIL Of the action of the veins\. These vessels, whose functions it is 

 to carry back to the heart, the blood which the arteries have sent to all 

 the organs, are much more numerous than the arteries themselves. It is 

 observed, in fact, that arteries of a middle size, as those of the leg a^ 

 fore-arm, have each two corresponding veins, whose calibre, at leS^ 

 equals theirs, and that there is besides, a set of superficial veins, lying 

 between the skin which covers th*> limbs and the aponeuroses which en- 

 velope the muscles: these have no corresponding arteries. The space 

 which the venous blood occupies is, therefore, much greater than that 

 taken up by the blood in the arteries. Hence also, it is estimated, that of 

 twenty-eight or thirty pounds of this fluid,, making about a fifth part of the 

 whole weight of the body in an adult man, nine parts are present in the 

 veins, and only four in the arteries. In this calculation, one should con- 

 sider as arterial, the blood contained in the pulmonary veins and in the 

 left cavities of the heart, while that which fills the cavities in the right 

 side of the heart and the pulmonary artery is truly venous, and has every 

 character of such blood. 



Although the veins generally accompany the arteries, and are united 

 to them, by a common sheath of cellular membrane, this disposition of 

 parts is not without exceptions. The veins which bring back the blood 

 from the liver, do not, in any respect, foliovv the course of the branches 

 of the hepatic artery : the sinuses of the brain are very different, in their 

 arrangement, from the cerebral arteries ; the veins of the bones, which 

 are particularly numerous, and of a much greater calibre than the arte- 

 ries of the same parts, from the slow circulation of the blood along them, 

 do not generally follow the direction of the arteries, and arise singly from 

 the substance of the bone, with the exception of those in the middle ca- 

 nal, and which pass through the nutritious foramen of the bone. The 

 veins are not only more numerous than the arteries, but they are like- 

 wise more capacious, and dilate more readily; this structure was neces- 

 sary, on account of the slowness with which the blood circulates, and of 

 the readiness with which it stagnates, when the slightest obstacle impedes 



* See APPENDIX, NoteN. 



-f See APPENDIX. Note T. 



t In the month of February, 1823, while preparing a subject for the demonstration of 

 the arteries, I was presented with the most satisfactory evidence of the direct communi- 

 cation between the arteries and veins. Into the left carotid artery of an adult and ema- 

 ciated subject, I introduced and secured a large sized pipe, pointing 1 towards the chest. 

 Into this was thrown with the ordinary syringe, melted tallow, highly coloured with 

 King's yellow. The injection was generally minute, more particularly so in the arms, 

 and most in that of the left side. In the arms, not only were the arteries filled, but the 

 veins of the smallest size, from the extremities of the fingers to the axilla, were entire- 

 ly distended. What was still more interesting, the tallow in the arteries was of a very 

 bright yellow, increasing in colour as the arteries became smaller, while in the veins the 

 tallow was entirely destitute of colour, having passed through vessels too small to allow 

 the colouring matter to accompany it. Wherever a small artery could be seen a vein 

 filled with the pure tallow could<be distinctly observed. 



This injection was demonstrated before a large class, and was shown to several mem- 

 bers of the Academy of Natural Sciences of Philadelphia. Godman* 



