176 EEPTILIA. 



open. They are lined internally by a raucous membrane, which is 

 covered with a net-work of flat cells and depressions, that secrete a 

 fetid kind of grease. An epithelium provided with the same cellular 

 surface rests quite freely upon the mucous membrane of these Anal 

 sacs. The Chelonia possess similar but more rounded anal sacs. 

 The Crocodiles have a thicker walled pouch situated beneath the 

 integument upon the middle of the lower jaw, and which is called 

 the musk gland, from its secreting a dark-colored grease smelling 

 like musk. 



The most remarkable peculiarity in the secretory organs of the 

 present class is afforded by the Poison glands, which occur, however, 

 only in the order Ophidia. 



The Poison-gland corresponds in some measure with the parotid 

 salivary gland, and agrees most strikingly in situation with the 

 latter in the Venomous Serpents, having posterior venom-teeth, e. g. 

 Dipsas, Homalopsis, where it lies more freely, not invested by any 

 fibrous tunic, and has a short excretory canal. In the typical 

 Venomous Serpents, as Vipera, Naja, Crotalus, Trigonocephalus, the 

 poison-gland is situated more behind and beneath the eye, consisting 

 of short tubes in Naja, or of hollowed ramified lobules (Trigono- 

 cephalus) and is surrounded by a dense, mostly double fibrous sheath, 

 this again being covered by a layer of muscular fibres, which 

 proceed partly from the temporal muscle, and serve to compress the 

 gland and force its contained secretion into the excretory ducts ; 

 the latter courses along the external surface of the superior max- 

 illary bone, and enters an opening placed at the root of the poison- 

 tooth. The situation and structure of the poison-gland are similar 

 in the Aquatic Serpents (Hydrus and Hydrophis). This gland 

 occurs in an unusual situation in Causus rhombeatus, being ensi- 

 form, situated in a channel-like cavity, and extending to the 18th 

 or 19th vertebra, so that it reaches over more than a seventh part 

 of the whole length of the body ; its excretory duct extends from the 

 poison-tooth to behind the quadrate bone. The ejection of the 

 poison into a living animal is accompanied by peculiar and frequently 

 fatal effects. 



ORGANS OF GENERATION. 



REPTILES, like all the Vertebrata, have two distinct sexes, which 

 exist in tolerably equal numerical proportion to each other, though 

 with some preponderance upon the part of the female. The germ- 

 preparing sexual organs are always situated within the abdomen, 



