POISONOUS SNAKES OF NORTH AMERICA. 



369 



and less developed, until the situation of the last Avhicli is visible is 

 marked by a minute papilla i>,lone. I have counted from 8 to 10 of 

 these on each side. When the fang is lost by natural process, it is 

 replaced within a few days: whei; violently displaced, several weeks 

 sometimes elapse before the next fang is fixed firmly enou<>h to be useful 

 to the sitake. If the functional fang be lost or shed, the next tooth 

 gradually assumes its position,'' and finally, occupying the j^lace of the 

 lost one, becomes anchylosed. 



In the same manner as the hollow fang is developed from the grooved 

 fang, and this again from the plain solid tooth, so is the poison gland 

 evolved fiom the ordinary salivary gland by a specialization of the 

 yellow portion of the latter, as already mentioned under the opistho- 

 glyph snakes. 



The typical venom gland is found in its fullest development among 

 the Pit Vixjers, and is located on each side of the head below and behind 



d g 



e b t 



Fit;. 18- 



POISON APPARATUS OF RATTLESNAKE; VENOM GLAND AND MUSCLES. 



Lateral view, a venom jiland ; a' venom duct; h anterioi- temporal mu.scle; 6' maudibiilar j)ortion 

 of same; c posterior tem])oral muscle; d digastricus Tiiuscle; c posterior ligament of glaud ; y'slieatli 

 of iang; g middle temporal muscle; /t external pterygoid muscle; i maxillary salivary gland; j man- 

 dibulary salivary gland. 



(After Duvernoy. ) 



the eye. The shape is that of a flattened almond, the pointed end 

 toward the front and below the eye, tapering to a narrow duct, which 

 carries the poison to the inlet at the base of the fang. The relative size 

 of the organ may best be understood by a glance at the accompanying 

 figure (fig. 18.) 



The interior structure of the gland may be described briefly as con- 

 sisting of a basal cavity into which the small ducts of the glands open. 

 These ducts run toward the walls of the gland, branching, and finally 

 ending in minute blind bags, the whole system of ducts being sup- 

 ported by a network of numerous fine threads and thin sheets of fil)rous 

 tissue. The ducts are lined with a more elongated epithelium, the blind 

 pouches with angular nucleated epithelial cells. 

 H. Mis. 184, pt. 2 24 



