ANATOMY OF THE FROG 51 



The cceliaco-mesenteric artery, which arises from the left 

 arch just at its point of union with its fellow, soon divides 

 into two branches, the coeliac and the mesenteric. The 

 coeliac artery is further divided into the gastric artery supply- 

 ing the stomach, and the hepatic, supplying the liver. The 

 mesenteric artery divides into an anterior and a posterior 

 branch and into the splenic artery, which goes to the spleen. In 

 its further course backwards the dorsal aorta gives off from 

 four to six small vessels from its ventral side, to supply the 

 kidneys and generative organs, and some small vessels from its 

 dorsal side, which supply the muscles of the back and sides. 

 These are the uro-genital and lumbar arteries. Opposite the 

 middle of the urostyle the dorsal aorta bifurcates to form the 

 iliac arteries which are continued onwards as the sciatic 

 arteries into the hind limbs. 



The third or pulmo-cutaneous arches divide, after a short 

 course on either side of the oesophagus, into the pulmonary 

 artery, which supplies the lungs, and the cutaneous artery, 

 a large vessel running up to pass close behind the ear, and 

 then turning sharply backwards to be distributed over the 

 skin of the trunk. 



The principal arteries which have been detailed above 

 divide and subdivide in the various regions of the body to 

 which they are distributed ; the walls of their branches 

 become thinner and less elastic, till finally they end in net- 

 works of extremely fine vessels with very thin walls, called 

 capillaries. The capillaries may easily be studied, by the aid 

 of the microscope, in the web of the frog's foot when the toes 

 are stretched apart. They present the form of a network of 

 fine tubes running through the tissues of the web. The 

 meshwork is irregular, but, speaking generally, the capillaries 

 form numerous loops of various sizes, and the capillary tubes 

 themselves unite freely with one another. Their walls are so 

 thin that they are hardly apparent, but the blood stream 

 coursing through their cavities is readily recognisable. The 

 capillary tubes vary in diameter from aoVo^h .to xsVcjth of an 

 inch. 



The capillaries are continuous, on the one hand, with the 

 ultimate subdivisions of the arteries. On the other hand, they 

 unite to form larger, but still very small vessels, the ultimate 

 subdivisions of the veins, also called venules. Both arterioles 



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