888 



TEETH. 



the fang of the large Cobra in my " Odonto- 

 gvapliy " (pi. 65. fig. 9., and in Vol. IV. p. 290. 

 Jig. 210., art. REPTILIA), where a fine hair is 

 represented as passing through the poison- 

 canal. 



The poison-glands occupy the sides of the 

 posterior half of the head ; each gland con- 

 sists of a number of elongated narrow lobes, 

 extending from the main duct, which runs 

 along the lower border of the gland upwards 

 and slightly backwards : each lobe gives off 

 lobules throughout its extent, thus presenting 

 a pinnatifid structure ; and each lobule is 

 subdivided into smaller secerning caeca, which 

 constitute the ultimate structure of the gland. 

 The whole gland is surrounded by a double 

 aponeurotic capsule, of which the outermost 

 and strongest layer is in connection with the 

 muscles by whose contraction the several ceeca 

 and lobes of the gland are compressed and 

 emptied of their secretion. This is then con- 

 veyed by the duct (see REPTILIA, Vol. IV. 

 p. 291., j%. 211. e) to the basal aperture of the 

 poison-canal of the fangyi We may suppose, 

 that as the analogous lachrymal and salivary- 

 glands in other animals are most active during 

 particular emotions, so the rage which stimu- 

 lates the venom-snake to use its deadly weapon 

 must be accompanied with an increased secre- 

 tion and great distension of the poison-glands ; 

 and as the action of the compressing muscles 

 is contemporaneous with the blow by which 

 the serpent inflicts the wound, the poison is 

 at the same moment injected with force into 

 the wound from the apical outlet of the per- 

 forated fang. 



The duct which conveys the poison, al- 

 though it runs through the centre of a great 

 part of the tooth, is really on the outside of 

 the tooth, the canal in which it is lodged 

 and protected being formed by a longi- 

 tudinal inflection of the dentinal parietes 

 of the pulp-cavity. This inflection com- 



Fig. 567. 



longitudinal indentation on the convex side 

 of the fang; as it proceeds it sinks deeper 

 into the substance of the tooth, and the sides 

 of the groove meet and seem to coalesce, so 

 that the trace of the inflected fold ceases, 

 in some species, to be perceptible to the 

 naked eye ; and the fang appears, as it is 

 commonly described, to be perforated by the 

 duct of the poison-gland. In the Hydrophis 

 the groove remains permanently open, as in 

 Jig 567. c. 



From the real nature of the poison-canal 

 it follows that the transverse section of the 

 tooth varies in form in different parts of the 

 tooth ; at the base it is oblong, with a large 

 pulp-cavity of a corresponding form, with an 

 entering notch at the anterior surface ; farther 

 on the transverse section presents the form 

 of a horse-shoe, and the pulp-cavity that of 

 a crescent, the horns of which extend into 

 the sides of the deep cavity of the poison- 

 fang : a little beyond this part the section of 

 the tooth itself is crescentic, with the horns 

 obtuse and in contact, so as to circumscribe 

 the poison-canal ; and along the whole of 

 the middle four-sixths of the tooth, the section, 

 of which a magnified view is given in Jig. 568., 



Fig. 568. 



Poison-fangs of Serpents. (Magnified.) 



mences a little beyond the base of the tooth, 

 where its nature is readily appreciated, as the 



Section of poison-fang of Serpent. (Magnified.) 



shows the dentine of the fang inclosing the 

 poison-canal, and having its own centre or 

 pulp-canal (p,p), in the form of a crescentic 

 fissure, situated close to the concave border of 

 the inflected surface of the tooth, The pulp- 

 cavity disappears, and the poison-canal again 

 resumes the form of a groove near the apex 

 of the fang, and terminates on the anterior 

 surface in an elongated fissure. 



The venom-fangs of the viper, rattle-snake, 

 and the Fcr-de-Iance are coated only with a 

 thin layer of a subtransparent and minutely 

 cellular cement. The disposition of the den- 

 tinal tubes is obedient to the general law of 

 verticality to the external surface of the tooth ; 

 it is represented as seen in the transverse sec- 

 tion from the middle of the fling in Jig. 56S. 

 Since the inflected surface of the tooth can 



poison-duct there rests in a slight groove or be exposed to no other pressure than that of 



