274 



THE VITAL FUNCTIONS. 



357 H 



shown in the diagram, Fig. 357. The heart of 

 the Frog, for example, may be considered as 

 consisting of a single auricle 

 (d), and a single ventricle 

 (e).* From the latter there 

 proceeds one great arterial 

 trunk, which is properly the 

 aorta. This aorta soon di- 

 vides into two trunks, which, 

 after sending branches to the 

 head and neck, bend down- 

 wards (as is seen at o, p), 

 and unite to form a single 

 trunk (a), which is the de- 

 scending aorta. From this vessel proceed all 

 the arteries which are distributed to the trunk 

 and to the limbs, and which are represented 

 as situated at b : these arterial ramifications are 

 continued into the great venous trunks, which, 



* Dr. Davy has observed that although the auricle appears 

 single, when viewed externally, its cavity is in reality divided 

 into two compartments by a transparent membranous partition, 

 in which some muscular fibres are apparent : these communicate 

 with the cavity of the ventricle by a common opening, provided 

 with three semilunar valves. Edin. Phil. Journal; xix, 161. 



Mr. Owen informs me that his own observations confirm 

 those of Dr. Davy ; and that he has discovered that the Siren 

 has also a distinct pulmonic auricle ; whence he infers that 

 wherever lungs are suflfiiciently developed to effect a change in 

 the blood, that fluid is conveyed to the ventricle by a distinct 

 route, and the pulmonary veins thus defended from the pressure 

 of the blood accumulated in the right auricle. 



