224 GENERAL ANATOMY OF THE SANGUIFEROUS SYSTEM. 



— The direct communication between the arteries and veins, 

 is readily shown by the facility with which the veins may be 

 distended, by injection of the arteries of the extremities with 

 mercury. In many of the viscera, also, it is not unusual to fill 

 some of the veins, when distending the arteries with size 

 injection. 



— Some anatomists (vide note, p. 216) assert that they have 

 in the human subject, been able to trace with the microscope, 

 the direct continuity of the arteries with the veins. This may 

 be done more readily in the inferior animals, especially in the 

 reptilia, where the globules of the blood are three or four times 

 larger than those of man. 



— In the dissection of two large serpents, a python from South 

 America, and a boa constrictor from the East Indies, made jointly 

 by the late Dr. John P. Hopkinson and myself, an account of 

 which was read before the American Philosophical Society in 

 1833, we were enabled by the aid of the mercurial pipe and 

 column, to pass quicksilver, in many instances, from a branch of 

 the aorta, into the capillary' arteries of the peritoneal investment 

 of the oviducts. Then by gently pushing forwards the minute 

 columns of mercury through these capillaries with the handle of 

 a scalpel, we found it to pass into the veins, and back again in 

 a retrograde direction to the vena cava. In these cases, the 

 capillary vessels when- distended with mercury, were very obvious 

 without the aid of a microscope. In the transparent parts of 

 animals, as in the mesentery, in the fins of some fishes, and in the 

 web of a frog's foot, or in a frog's tongue, the microscope exhibits 

 very clearly the capillary system of vessels, as well as the series 

 of blood globules which pass through them from the arteries to 

 the veins. 



— Fig. 174, is a representation, after Thomson,* of the appear- 

 ance of the capillary circulation in the web of a frog's foot. 

 The arrows indicate the course of the blood, the globules of 

 which are seen in these vessels arranged in a linear series. 

 — Marshall Hall, considers the capillary system as a distinct 

 portion, reticular in its structure, the branches anastomosing 



* Circulation — Cyclop, of Anat. and Physiology. — 



