304 ZOOLOGY SECT, xin 



to the right auricle. The auricles have their inner surfaces raised 

 up into a network of muscular ridges, the musculi pectinati. 

 Both auricles open into the cavity of the ventricle, the aperture of 

 communication, or auricula-ventricular aperture, being divided into 

 two by the auricular septum, and guarded by the auriculo-ven- 

 tricular valve, consisting of two semi-lunar flaps. The ventricle 

 (Fig. 913, v.] Fig. 914, vent.) has very thick spongy walls, and 

 a small cavity, divided into two parts by an incomplete mus- 

 cular partition. From the part of the ventricular cavity to 

 the right of the partition arises the pulmonary artery ; from the 

 part to the left are given off the right and left aortic arches. 

 When the two auricles contract, the blood from the right 

 auricle (venous blood) tends to run more to the right-hand 

 portion of the cavity of the ventricle, while that from the left 

 auricle (arterial) occupies the left-hand portion. When the 

 ventricle begins to contract its walls come in contact with the 

 dorsal and ventral edges of the ventricular partition, thus com- 

 pleting the separation of the right-hand part of the cavity, 

 containing venous blood, from the left-hand part, containing 

 arterial and mixed blood; and the further contraction results 

 in the driving of the venous blood through the pulmonary artery 

 to the lungs and of the rest through the aortic arches to the head 

 and body. (Vide Fig. 945.) 



From the right aorta rise the carotid arteries (Fig. 913, cr. ; 

 Fig. 914, car. art.), and each runs for some distance parallel with 

 the corresponding aortic arch, with which it anastomoses distal ly 

 (the connecting part being termed the ductus Botalli), having 

 previously given off the carotid artery proper, by means of which 

 the blood is carried to the head. The two aortic arches curve 

 backwards round the oesophagus, the one on the right hand and 

 the other on the left, and meet in the middle line dorsally to 

 form the median dorsal aorta (Fig. 913, d. ao. ; Fig. 914, dors. aort). 

 From the right arch, just in front of the junction, arise the two 

 subclavian arteries, right and left, each running outwards to the 

 corresponding fore-limb. From the dorsal aorta the first im- 

 portant branch given off is the cceliaco-mesenteric (Fig. 913, c.'m.). 

 This shortly divides into two trunks, a ccdiac (Fig. 914, cod. a.) 

 supplying the stomach, spleen, pancreas, duodenum, and left lobe 

 of the liver, and an anterior mesenteric supplying the posterior 

 part of the small intestine. Three small posterior mesenteric 

 arteries given off further back supply the large intestine. Pos- 

 teriorly, after giving off renal and genital branches, and a pair of 

 large iliacs to the hind-limb, the dorsal aorta is continued along 

 the tail as the caudal artery (Fig. 914, caud. art). Throughout 

 its length, in addition to the larger branches mentioned, the dorsal 

 aorta gives off a regularly arranged series of pairs of small 

 vessels, the intercostal and lumbar arteries, giving off branches 



