216 CLASS XV. 



cavce, wliicli here, in the absence of subclavian veins, are formed 

 by the jugular veins, and one posterior or ascending vena cava, 

 return in the serpents the venous blood to the hearth 



In the lizards and tortoises the heart is formed of two auricles, 

 usually completely separate from each other by a septum, and two 

 ventricles only incompletely separate from eacli other, with the 

 exception of the crocodiles. However in the tortoises, according to 

 Caldesi and others, the arterial blood is distinguished by a bright 

 red colour from the darker venous blood ^. In the tortoises the 

 heart is broader than long, and has very spacious auricles and thick 

 muscular walls. From the heart arise two arterial stems or in 

 some a single stem, which, however, divides immediately after 

 leaving the heart, into a right and left stem. Of these the right 

 stem is the thicker and divides into a common truncus anonymus 

 and a descending stem ; the first divides into a left and a right 

 branch, from which the subclavian and jugular arteries of each 

 side arise. The descending stem forms on the right side an arch, 

 and afterwards unites with a branch from the left stem to form 

 the descending aorta. Before giving off the connecting branch, 

 which may be regarded as the continuation of the stem, the left 

 aorta supplies arteries for the stomach and the mesentery^. The 

 pulmonary artery divides into two branches ; between each of these 

 branches and the arch of the left and right aortte there is a com- 

 municating branch [ductus Botalli) , which, however, in the tortoises 

 is consolidated and closed at an early period ; consequently, the pul- 

 monary artery becomes suddenly smaller before it arrives at the 

 lung. In the lacertine animals also the left aorta gives off no 

 branches forward, to the head or anterior limbs, but only to the 

 intestines, except a communicating branch to the descending right 



^ F. ScHLEMM Anatom. Beschre'ihung des Bluttgefdss-Systems der Schlangen, in 

 TiEDEMANN u. Treviranus Zeitschrift fur Physiol, ii. s. loi — 124, Taf. vii. On the 

 circulation of sei-pents see H. Jacquart Mem. sur la circulation chez le Python, Ann. 

 des Sc. nat. 4ifenie S^rie, iv. 1855, pp. 3-21 — 364, PL 9 — 11, H. Rathke Bemerhungen 

 iiber die Caret iden der Schlangen, Wien, 1856 {Denkschr. der mathem. nalurw. Classe 

 der kaiseil. Akad. der Wissensch.). 



2 BiAJUK^BKcn's, Kleine Schriften. Leipzig, 1804, Svo, s. 70, 71. 



3 Hence Cuvier names the left cwHa also aorte viscercde. See figures of the heart 

 and large vessels in Emys europcea in the classical work of Bojanus, Tab. xxix. figg. 

 160, 161. 



