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

 of urinary tubule ; glm. 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 ; 1. au. left auricle ; Lng. lung ; Lvr. C. liver-cells ; ly. cp. lymph 

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

 stome ; p. 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. an. right auricle ; rn. pt. v. renal portal vein ; scl. a. subclavian artery; 

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

 artery ; spl. a. splenic artery ; sp 1. v. splenic vein ; s. v. sinus vcnosus ; 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. 



contained blood, which, coming by the precavals and 

 postcaval, is non-aerated, is acted upon in all directions 

 and might therefore be forced either into the three great 

 veins (pr. cv. v, pt. cv. v) or into the right auricle (r. au). 

 But the veins are full of blood steadily flowing towards 

 the heart, and any regurgitation is further prevented by 

 their valves : the right auricle, on the other hand, has 

 finished its contraction and is now relaxing ; it is there- 

 fore 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 re- 

 filled 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 from the sinus, the left (L 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. Con- 

 sequently 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 



