524 ANATOMY OF VERTEBRATES. 



lung : it sends a branch to the ' rete mirabile ' of the parotoid, or 

 group of subcutaneous cervical glands, and, in the Frog, the 

 branch from the pulmonary artery joins one from the aorta to 

 form a large subcutaneous artery, more extensively distributed 

 upon the skin, which exercises a respiratory office in these 

 naked Batrachia. In the Siren, Menopome, and Amphiume, 

 the pulmonary artery distributes some small twigs to the oeso- 

 phagus. 



In the poisonous, colubrine, and marine Snakes the lung is, 

 functionally, single, one only being developed, but of great length, 

 for the respiratory purpose, the other becoming aborted into a 

 small or scarcely discernible rudiment. The trachea, fig. 300, a, 

 has numerous and entire cartilaginous rings along a certain pro- 

 portion of its course, and the lung, ib. B, seems to be formed by 

 a dilatation of the membranous part of the tracheal tube, after 

 the rings become incomplete : but these may be traced sujiporting 

 a narrow and shallow kind of air-canal to the hind end of the 

 luno' in Pclamis bicolor.^ A similar structure has been observed 

 in Vijiers {^Echidna arietans), and, for a shorter extent, in Kattle- 

 snakes, in which the trachea is shorter than in non-venomous 

 Snakes. The expanded membranous part of the lung is honey- 

 combed, Avith subdivisions of the alveoli at the fore-part of the 

 lung, for a varying thickness and extent in different species. The 

 rudiment of the atrophied lung is often indicated by an orifice in 

 the trachea, at or near its entry into the functional lung, leading 

 to a small pouch. This adheres to the terminal whole-rings of 

 the trachea in Coluber natrix and Naja tripudians. In the great 

 constricting Serpents both lungs are functionally Init Tincqually 

 developed. In Pijthon tigris the left lung is nearly half as lono- 

 as the right : but, by its structure, it takes almost an equal share 

 in the respiratory function, the vascular and honeycombed parietes 

 being of nearly the same extent as in the right luno;. So much 

 of this as is prolonged beyond the left lung has thin, simple, and 

 comparatively unvascular walls, performing the office of a reservoir 

 of air, which may be useful during the period, when the delicate 

 windpipe is squeezed flat by a large prey in progress of slow 

 deglutition.^ The proper parietes of the lungs almost every- 

 where adheres, by lax cellular tissue, to the contiguous oro-ans. 

 In the Slow-worm {Aiir/uis fraflil/s) the lungs are relatively 

 shorter than in true Ophidia, but the left is only half the length 

 of the right lung. A similar diftercnce is presented by the lungs 



' xx. vol. ii. p. 93, iircp. no. 1089. '' Ib. p 94, prep, no 1093 A. 



