362 OPHIDIA. 



and sometimes dorsally by transverse pieces. There are no 

 vocal cords. The trachea is long and the tracheal rings are com- 

 plete in front, incomplete behind. The lungs are saccular and 

 provided with air-cells in their walls. They are never sym- 

 metrical. As a rule the left is the smaller, and in some cases, 

 e.g. most poisonous snakes, one lung is completely suppressed. 

 In such cases the posterior end of the trachea may possess, 

 usually on its dorsal side, air-cells in its walls and take^on the 

 structure of a lurig. In many snakes the alveolar structure is 

 less marked and may be entirely absent in the hind end of the 

 lung. It thus happens that in these snakes, as in some lizards 

 (Chamaeleo), the hind end of the lung has a smooth wall without 

 lung structure and is supplied with blood vessels from the sys- 

 temic circulation. Inspiration is effected by swallowing move- 

 ments as in Amphibia and lizards. 



The brain is very similar to that of lizards, but there is no 

 parietal organ. The spinal accessory cranial nerve is not pre- 

 sent, and the sympathetic system is but feebly developed, the 

 viscera in the greater part of the trunk being supplied by the 

 ventral rami of the spinal nerves. 



The eyelids are always fused over the eye, so that the conjunc- 

 tival space is closed and there appear to be no eyelids (see p. 317). 

 The lacrymal gland opens into this space, which also communi- 

 cates with the nose by a lacrymal canal. There is no retractor 

 bulbi muscle. 



Tympanic membrane, tympanic cavity and eustachian tube 

 are absent. In spite of this snakes appear to have a good sense 

 of hearing. The columella auris extends from the stapedial 

 plate to the quadrate, against which it abuts by a cartilaginous 

 epiphysis. In some snakes the stapes is a bony plate closing 

 the fenestra ovalis and without a shaft-like columella. 



The nasal openings are usually placed at the front end of the 

 snout. 



The heart is placed far back at some distance from the head. 

 It is constructed as in lizards and chelonians. There are a sinus 

 venosus, two auricles, and a single ventricle imperfectly divided 

 by a septum. Three arteries leave the ventricle, the pulmonary 

 artery and two systemic arches. The right systemic arch 

 gives off the single or double carotid artery. In many snakes 

 (Python, Boa, Tropidonotus, Xenopeltis, etc.) there are two 



