568 



KEl'ORT OF NATIONAL MUSEUM, 1893. 



Fig. lij. 



1>IA(;RAM ok the boxes CONCKKNKD in KAISL.Ii 

 THE FANG. 



a Pterygoid boue; ^)rt arrow markiug its lir.p 

 of'motion ; ;j-e external pteryjcoifl muscle ; (j t'routal 

 bone; rf lachrymal bone; c mamillary hone; M'ang. 



I AHer Mitchell. ) 



The fiiii.ii- itsdf is ji hirjie, very i)ointe(l, and (mrved tootli coutaiiiiiig 

 two cavities, the pulp cavity and the poison canal, the tonner situated 

 on the concave side, the latter on the convex side of the tooth ((!<;•. 20), 

 Tlie poison canal has a more or less slit-shaped openinj^ ncjir the base, 

 oil the anterior side of the fang, and another slit, .narrower and longer 



on the same side, some little dis- 

 tance from the very sharply 

 pointe<l tij). Between these open- 

 in jjs it is often possible to trace a 

 more or less well-defined dejnessed 

 line. A microscopic inspection of 

 cross sections of the fang reveals 

 the fact that the canal is nothing 

 but a deep groove, the walls of 

 which have closed over it ante- 

 riorly, the depressed lineindicating 

 the meeting of the walls, or the 

 "seam." This structure of the fang 

 may be easily understood by com- 

 paring it to a leaf curling up in drying, the edges meeting and over- 

 lapping in the middle, leaving anuppsr and a lower opening. By making 

 sections of growing and full grown fangs of the same individual, the 

 evolution of the grooved fang into the " perforated" fang is easily traced, 

 and the inexactness of the latter term clearly demonstrated. As a 

 consequence of this origin of the canal, it is lined with the same hard 

 layer of dentine as the outer side of the fang, for it will be seen that 

 this inner lining of the canal is in reality the 

 anterior surface, while the outer layer is only 

 the posterior surface of the normal tooth. An 

 inspection of the accomjoanying figure (fig. i7), 

 which represents a cross section through the 

 upper jaw, with the functional full-grown fang 

 followed by the moreor less undeveloped reserve 

 fangs, will show this plainly, as the same prin- 

 ciple is involved in the Pit Vipers. 



As already stated, the fang is above fixed 

 firmly in the socket of the maxillary bone, "its 

 base being luted to the jwrtion of the bone 

 around its side and anterior aspect," to borrow 

 Dr. Weir Mitchelfs words.* "Posteriorly, the 

 bone possesses a hollow, in which is lodged the 



tooth sac. In the open mouth of this alveolar process, within the raucous 

 membrane, and upon the pterygoid bone, lie one l>ehind ami below 

 another, the reserve fangs, each smaller than the one in front an<l less 



Fig. 17. 



A.:?fiAN«EMENT OP THE RESERVE 

 FANGS IN Br.N(;AKU!S SE.MIFAS- 

 CIATUS, ILLUSTHATtNG THE DE- 

 A-ELOPMENT OF THE GROOVEH 

 FANG INTO THE CANALED FANG. 



Enlarji.-.l. 



'Researches upon tin- Vouom of the Rattlesnake. Smithsonian C'outrilmtions to 

 Knowledge. Washington, 1861, p. IH. 



