VOL. XXIII.] PHILOSOPHICAL TKANSACTIONS. (383 



those animals, on which only these experiments have been hitherto made, in- 

 duced me to make some on animals whose organs differ only from the human 

 in their gross figure, and not in their intimate structure ; for this end I took a 

 young cat about 10 or 12 days old, and fastened it to a board as in vivesection ; 

 and making an incision through the linea alba, the omentum and intestines 

 were extruded ; then causing the animal to be so held on the board, under a 

 large double microscope, where a fiat glass for receiving of objects was placed 

 horizontally, on which I expanded the omentum or caul, a light being placed 

 underneath, I saw the globules of the blood move very swiftly in the small 

 vessels, which are only to be seen in the most transparent parts of the mem- 

 branes of the omentum ; but the motion of the blood soon abated, and its 

 globules were withdrawn from the extremities of its blood vessels, and in a little 

 time became stagnant in their larger branches. 



This appearance of the continuation of the extremities of the arteries, with 

 the veins, while the blood was moving in them, I saw very well in the omentum 

 of a young lean dog, that was not large ; but by the assistance of an instrument 

 I had prepared to expand the mesentery, we all saw it there much better ; that 

 part having not only larger and clearer spaces than the omentum, but its blood 

 vessels distributed more regularly. 



Those who would view with microscopes the transparent parts of living ani- 

 mals, will find that the extremities of their arteries and veins are not all equally 

 lessened though united : in the tail of the lacerta aquatica, tadpoles, and in most 

 fish, I have frequently observed several communications between the arteries 

 and veins, in which more than two globules of blood have passed abreast : and 

 in the same area I have seen some of those communications so small that only 

 one globule could pass before the other, and that very slowly. In young fish, 

 particularly in grigs, I have frequently observed a communicant branch, so very 

 small, as that one globule of blood only has passed it in two or three seconds 

 of a minute ; at other times I have found considerable intervals in the passing of 

 one globule in such a communicant branch ; even half a minute, a whole mi- 

 nute, and once in two or three minutes I have seen one globule of blood only 

 pass in a particular tract. 



The ready passing of injections by the splenic arteries, to the veins, shows 

 that the communications between those vessels are more open than the arteries 

 and veins of other parts. As also injections into the pulmonic arteries pass to 

 their veins, though not altogether so freely, as in the spleen. 



On viewing with a microscope the extremities of the pulmonic blood vessels 

 in a live frog, I found their communications much larger than those that T had 

 before seen, in the membrane between the toes, and in the feet of that animal. 



4 s 2 



