330 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



supplied blood. The microscope has settled this question in the 

 affirmative, for most of the tissues and organs of the body: but it may 

 still be doubted whether the finest capillaries of the liver have walls 

 distinct from those cubical masses of epithelium which they permeate. 



Although it is the rule that an intermediate system of vessels exists 

 between the arteries and veins, we find two remarkable exceptions 

 to it. One is in the erectile tissue of the penis ; the other in the 

 uterine circulation. In both these instances, the arteries communicate 

 directly with the veins. In the penis, the ramifications of the arte- 

 ries pour their blood into the cells of the corpora cavernosa; in the 

 uterus, the small curling arteries of Hunter open directly into large 

 venous sinuses, which, in the gravid uterus, form an intimate relation 

 of contact with the villous processes of the placenta. These points 

 will be fully described in the chapter on Generatior. 



It is not improbable that further research may detect a direct 

 communication between arteries and veins, even in tissues, the 

 greatest part of which is furnished with a true capillary plexus. 

 In the cancellated structure of bone, and the diploe of the cranial 

 bones, it seems highly probable that the arteries communicate 

 immediately with the veins at many points. Mr. Paget* describes 

 a direct communication between the arteries and veins of the wing 

 of the bat, without any intermediate capillary plexus.f 



* Lectures on Inflammation. 



t The communication between arteries and veins by capillaries, was not 

 known to Harvey. In his time, and for a long period afterwards, anatomists 

 supposed the blood to pass into the parenchyma of the tissues, whence it was 

 received or withdrawn by the veins. Malpighi (about 1687), by microscopic 

 examination, first demonstrated the intermediate capillary system in the lungs 

 and urinary bladder of the frog. Leuwenhoek afterwards (1729) pursued this 

 investigation, and has given some good illustrations of the capillaries examined 

 in transparent parts during life. 



Dr. Hales, in this country, many years later (1769), gave a very accurate de- 

 scription of these vessels, and denied altogether the idea of the intervention of 

 a parenchyma, or, in his own words, of " glandular cavities." See his Hsema- 

 statics, p. 146, 9, vol.ii. W. Cowper, the distinguished myotomist, also made 

 observations on the capillaries of the transparent parts of warm-blooded ani- 

 mals, as the mesentery of a dog, and the omentum of a cat. Haller threw 

 great light upon this subject, and by his facts and arguments, settled the ques- 

 tion as to the direct continuity of arteries and veins. Subsequently (1745), 

 Lieberkiihn advanced our knowledge of the capillaries by his numerous injec- 

 tions, most of which are still extant at Vienna. In more recent times, the 

 distinguished Prochaska seems to have been the first to form a just apprecia- 

 tion of the extent of the capillaries, and of their exact relations to the elements 

 of the tissues. His description of the disposition of these vessels, based upon 

 the examination of Lieberkiihn's and his own injections, can scarcely be sur- 

 passed in the present day. See the 9th chapter (de vasis sanguin. capillar.) 



ur- 



