78 INTRODUCTION 



of lower Vertebrates have been found in some of the 

 Boidae. 



The bifurcate transverse processes of the vertebrae 

 at the limit between the body and tail enclose the 

 lymph-hearts, which are large and more or less 

 elongate, metamerically divided into several cham- 

 bers, the right often more developed than the left. 

 The thymus gland lies on each side of the trachea, 

 near the heart, and the thyroid gland is in the middle 

 line, close to the base of the carotid artery. 



The trachea is long, and the tracheal rings may be 

 complete in front and incomplete behind, or incom- 

 plete throughout. The bronchus opens at once into 

 the more or less elongate, usually single lung, with or 

 without a rudiment of a second, which seems to be 

 constantly the left ; in some snakes the lung extends 

 nearly to the cloacal region. In most of the Boidae 

 there are two well-developed lungs, the left shorter 

 than thq right. The lung has highly cellular walls in 

 front, and becomes thin-walled, smooth, or but little 

 vascular, behind, where it may receive its blood from 

 the systemic and not from the pulmonary circulation. 

 In the Typhlopidae and Viperidae, as well as in 

 some of the Boidae, Colubridae, and Amblycephalidae, 

 the posterior end or the greater part of the trachea 

 may have its wall enlarged and provided with air cells, 

 resembling the normal lung, with which it is usually 

 continuous ; this has been called the " tracheal lung," 

 but, although serving as an accessory breathing organ, 



