92 THE FROG CHAP. 



or spermary ; Cp. Hd. of head ; Cp. H. L. of hind-limb ; Cp. Kd. of kidney ; 

 Cp. Lng. of lung ; Cp. Lvr. of liver ; Cp. Pn. of pancreas ; Cp. Sk. of skin ; 

 Cp. Spl. of spleen ; cu. a. cutaneous artery ; cu. v. cutaneous vein ; Cu. Gl. 

 cutaneous gland ; d. ao. dorsal aorta ; Ent. C. enteric canal ; Ep. Ent. 

 epithelium of enteric canal ; Ep. Lng. of lung ; Ep. Sk. of skin ; Ep. Ur. T. 

 of urinary tubule ; glrn. glomerulus ; il. a. iliac artery ; int. a. artery to 

 stomach and intestine ; int. v. vein from stomach and intestine ; ju. v. 

 jugular vein ; hp. a. hepatic artery (to liver) ; hp.pt. v.- hepatic portal vein ; hp. v. 

 hepatic vein ; /. au. left auricle ; Ln?. lung ; Lvr. C. liver-cells ; ly. cp. lymph 

 capillaries ; ly. v. lymphatic vessels ; Mlp. Cp. Malpighian capsule ; nst. nephros- 

 tome ; /. ly. ht. posterior lymph-heart ; Pn. C. cells of pancreas ; Pn. D. pan- 

 creatic duct ; pr. cv. v. precaval vein ; pt. cv. v. postcaval vein ; pul. a. pul- 

 - monary artery ; pul. cu. tr. pulmo-cutaneous trunk ; pul. v. pulmonary vein ; 

 r. au. right auricle; rn.pt. v. renal portal vein; set. a. subclavian artery; 

 s. c. ly. s. sub-cutaneous lymph-sinus ; scl. v. subclavian vein ; s/>. a. splanchnic 

 artery ; spl. a. splenic artery ; spl. v. splenic vein ; s. v. sinus venosus ; syst. tr. 

 systemic trunk ; U. Bl. urinary bladder ; Ur. ureter ; v. valve in vein ; vs. a. 

 vesical artery (to bladder) ; vs. v. vesical vein ; vl. ventricle. 



auricle, on the other hand, has finished its contraction and is 

 now relaxing ; it is therefore empty. Thus, on the principle 

 of least resistance, the contraction of the sinus fills the right 

 auricle with blood from the great veins, and the sinus itself 

 is refilled from the same source as soon as it begins to relax. 



Immediately after the sinus has ceased to contract the 

 two auricles contract together : the right, as we have seen, 

 has just been filled ftom the sinus, the left (/. au] is full of 

 aerated blood brought to it by the pulmonary vein (pul. v). 

 The presence of the sinu-auricular valves prevents the blood 

 in the right auricle from being forced back into the sinus : 

 that in the left auricle is prevented from being forced back 

 into the pulmonary veins by the steady onward flow in the 

 latter. On the other hand the ventricle is beginning to 

 relax and is empty. Consequently the auriculo-ventricular 

 valves are forced back into the ventricle (vl) and the blood 

 from both auricles flows into and fills that chamber, the 

 right half of which becomes filled with non-aerated, the left 

 with aerated blood, the two taking an appreciable time to 

 mingle. 



The instant it is thus filled, the contraction of the ventricle 

 begins. As it does so the blood, getting behind the auriculo- 



