314 



ZOOLOGY 



SECT. 



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Vascular System. The heart is enclosed, like that of the 

 Frog, in a thin transparent membrane, the pericardium. It consists 

 of a sinus venosus, right and left auricles, and an incompletely divided 

 ventricle. The sinus venosus (Fig. 979, s. v.), into which the large 



veins open, is thin-walled, and has 

 a smooth inner surface. From it 

 a sinu-auricular aperture, guarded 

 by a two-lipped valve, leads 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 ven- 

 tricle, the aperture of communica- 

 tion, or auriculo-ventricular aper- 

 ture, being divided into two by the 

 auricular septum, and guarded by 

 the auriculo-ventricular valve, con- 

 sisting of two semilunar flaps. The 

 ventricle (Fig. 979, v. ; Fig. 980, vent.) 

 has very thick spongy walls and a 

 small cavity divided into two parts 

 by an incomplete muscular 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 

 FIG. 978. tacerta agiiis. General left-hand portion. When the ven- 



rBlfit'ioiis Bl u rin IT v Ijliclclcr * Ci Dosfc- TJriClc DcSfllJ-S uO CO II ul <AO T/j 1 Lo Wdllo 



Madder '^"H heart ^Tver ^ g /"~ come m contact with the dorsal and 

 the lungs; M, stomach ; MD, small m- ventral edges of the ventricular par- 



testine ; Oe. oesophagus ; Pn. pancreas ; . . , , ,. , 



Tr. trachea. (From Wiedersheim's tltlOn, thus Completing the Separa- 

 tion 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. 1013.) 



From the right aorta arise the carotid arteries (Fig. 979, cr. ; 

 Fig. 980, car. art.), and each runs for some distance parallel with 



