272 THE BIOLOGY OF THE FROG chap. 



is given off to the skin and muscles of the anterior part of 

 the thigh. Distal to the origin of the femoral artery the iliac 

 artery is continued as the sciatic, which passes out of the 

 body cavity, along with the sciatic nerve, just behind the 

 posterior end of the crest of the ileum, and supplies the hind 

 limbs. 



The third branch of the truncus, the piilmo-cutaneoiis 

 artery, passes outward and dorsally, and gives off posteriorly 

 \ht pi/lmoimry artery to the lungs; it is then continued as 

 the great cutaneous artery, which passes upward and forward, 

 dividing behind the tympanic membrane into three branches : 

 (i) the auricularis, which supplies the tympanum and gives 

 branches to the thymus, lower jaw, pharynx, and hyoid, and 

 anastomoses with branches of the occipital and internal 

 carotid; (2) the dorsalis, which is mainly distributed to the 

 skin of the back ; and (3) \h& lateralis, which passes outward 

 and is extensively^ distributed to the skin of the side of 

 the body. With the exception of a few minor branches the 

 distribution of the pulmo-cutaneous artery is such as to bring 

 the blood into regions where it may undergo oxygenation. 

 The blood which does not go to the lungs passes into the 

 great cutaneous artery, the largest branches of which are dis- 

 tributed to the skin, where much of the respiration of the frog 

 takes place ; the buccal cavity and pharynx, which are also 

 respiratory organs, receive blood from the same source. 



The Veins. — With the exception of the blood coming 

 from the lungs,_al]_of the blood is returned to the heart 

 through the three^Jarge venous trunks which enter the sinus 

 venosus.' The two anterior venae cavas, which enter the 

 anterior angles of the sinus, are each formed through the 

 junction of three branches, the external jugular, the innomi- 

 nate, and the subclavian. The most anterior of tliese, the 

 external jugular, passes forward, where it receives branches 



