384 



pt. z. 



Questions on the figure. Try to form a clear picture of the relations 

 of the articulating surfaces of the vertebne and indicate the possible ad- 

 vantages of the arrangements. Where is the neural cavity? Where do 

 the ribs articulate? What is the gain in muscular attachments from the 

 numerous bony outgrowths on the vertebrae? 



410. Respiration. Functional gills never occur, though 

 gill-slits are partly developed in the embryo only to close again 

 before hatching. The trachea is elongated and is supported 

 by cartilaginous rings as in the higher forms. It divides into 

 two bronchi, each of which passes to a spindle-shaped sac 

 the lung which is much simpler in its lobings than those of 

 birds and mammals. In the snakes one lung (the left) is much 

 reduced or even altogether aborted. This is an adaptation to 

 the narrow elongated body cavity. The ribs when present and 

 the muscles acting on them are the prime agents in breathing. 



411. Circulation. In reptiles the right and left auricles are 

 entirely distinct but, with the exception of the Crocodilia, the 

 ventricles are only partially so. Yet in those forms in which 

 the pure blood of the left auricle and the impure blood of the 

 right partially mingle in the ventricle, the arrangement is such 



