206 ANIMAL BIOLOGY. [Part I. 



The aortic arch passes down behind (dorsal to) the heart, and 

 supplies the trunk, alimentary canal, and hind limbs. 



The blood thus distributed throughout the body is returned 

 to the right auricle by three vessels, the postcaval (pt. c.) and the 

 right and left precavals (p. c.). The postcaval passes through the 

 substance of the diaphragm, and runs forward a little to the right 

 of the median plane. The precavals are constituted by the 

 external jugular (ex. ju.) (which nearer the head is seen to arise 

 from the union of two factors, an anterior and a posterior facial, 

 the latter being external), 1 the much smaller internal jugular 

 (in. ju.\ and the large subclavian (s. cl. v.). Nearer the heart each 

 receives an anterior intercostal vein (a. i. c. v.) (shown only on the 

 left side), collecting blood from the anterior intercostal spaces ; 

 and the right precaval receives also an azygos vein (az. v.) (probably 

 answering to the right posterior cardinal of the fish), which 

 collects blood from the posterior intercostal spaces. 



Traced backwards the aorta is seen to give off a large cwliac 

 artery to the stomach, spleen, liver, gall-bladder, and anterior 

 part of the duodenum ; and a little further down a still larger 

 anterior mesenteric artery supplying the greater part of the intes- 

 tines. Next are given off the renal arteries to the kidneys, and 

 then, some way further down the body, a small posterior mesenteric 

 artery (Fig. 64, p. m. a.), supplying the rectum. After giving 

 off the spermatic or ovarian artery (sp. a.) to the testis or ovary, 

 the aorta bifurcates to form the common iliac arteries (com. il. a.). 

 To follow these arteries the lower part of the innominate bones 

 should be removed, together with the rectum and genital organs. 

 As shown in the figure, they give off ilio-lumbar branches (iL I. a.) 

 to the abdominal walls of this region, and then bifurcate to 

 form (1) the external iliacs (ex. il. a.), which, after giving off 

 branches to the bladder (and uterus), become the femoral arteries 

 supplying the leg; and (2) the internal iliacs (in. il. a.\ which 

 pass along the dorsal wall of the pelvic cavity. One more artery 

 is to be noted, the median sacral (m. s. a.), which is drawn aside 



1 Occasionally the two external jugulars are united by a connecting trunk in 

 the posterior region of the neck. In the higher mammalia there is but one pre- 

 caval vein the right into which the external jugulars of both sides fall. 



