THE FROG 63 



The dorsal aorta then passes backwards to lie between the 

 kidneys, to which it gives off a branch on each side that in its turn 

 sends a genital artery (spermatic in the male and ovarian in the 

 female), running to the gonad and its accompanying fat body. 

 Immediately following this are from three to five renal arteries 

 passing off to the kidneys. In this part of its course also a few short 

 arteries, the lumbars, arising from the dorsal aorta, supply the 

 muscles of the back. Behind the kidneys the dorsal aorta gives 

 off a small posterior mesenteric and then divides into two large 

 trunks, the iliacs, one running towards each leg. After travers- 

 ing a short distance the iliac gives rise to a branch that almost 

 immediately divides into an epigastric, supplying the posterior 

 portion of the ventral body wall, and a recto- vesicular, supplying the 

 end of the rectum and the bladder. Soon after this the iliac splits 

 into a femoral, feeding the muscles and skin of the upper part of the 

 thigh, and a sciatic, that is distributed to the remainder of the hind 

 limb. 



The external carotid artery just beyond the carotid " gland " is 

 tied to the systemic arch by a thin strand of tissue. In the tadpole 

 this is an open tube, and in consequence it is termed the Ductus 

 caroticus, a similar remnant, only between the systemic and pul- 

 monary arches, is found in mammals, and there called the Ductus 

 Botalli, after its discoverer, Botallus. 



The Pulmo-cutaneous Arch, the hindermost of the three, also 

 passes round to the side of the oesophagus where the pulmonary 

 artery arises and runs along on the outer side of the .lung. The 

 great cutaneous artery, as the trunk is now called, is a large vessel 

 which after passing close to the ear is reflected backwards and 

 ramifies over the under surface of the skin. 



From the main arteries, briefly mentioned above, smaller arteries 

 and ultimately arterioles arise, and so constitute a network of vessels 

 running throughout all the various parts of the body, where, as 

 has been noted already, they break up into minute capillaries, and 

 so allow the blood to be distributed everywhere. 



Venous System. 



As we have already seen, the blood is collected up from 

 all over the body by a series of veins which unite to form larger 

 and larger trunks and, except the blood from the lungs ; it is all 

 returned to the sinus venosus b)' the three caval veins. 



The pulmonary veins are vessels running up the dorso-lateral 

 borders of the lungs and uniting to form a single vein which enters 

 the left auricle immediately in front of the anterior edge of the 

 sinus venosus. 



