PEOGRESS OF MICEOSCOPICAL SCIENCE. 231 



successional poison-fangs are developed in a single series, like any 

 other Ophidian teeth. 



The development of the individual tooth- germ presents one feature 

 of very great interest, A poison-fang tooth- germ is first formed, like 

 any other, of an extinguisher-shaped enamel-organ (derived from an 

 ingrowth of epithelium, which winds in and out amongst the tooth-sacs) 

 and of a simple conical dentine-pulp. 



As it elongates, a groove appears on one side, which, by deepening 

 and. by the approximation of its lips, becomes ultimately converted into 

 the poison-canal. The enamel-organ, with its characteristic enamel- 

 cells, passes without break or alteration into this groove ; but still 

 lower down is the tooth -germ, w^here the groove has become very deep ; 

 instead of the prismatic enamel-cells constituting a regular pavement 

 epithelium, we have a reticulum of stellate cells, just like that gelatinous 

 stellate tissue which forms so large a j)art of a mammalian enamel- 

 organ. That the stellate reticulum is a non-essential structure I have 

 previously shown ; but the occurrence of such a tissue within the 

 poison-canal, which it wholly occupies, and in which it represents the 

 prismatic enamel-cells found higher up, strongly suggests the idea 

 that it is a soi-t of retrograde metamorphosis of an active enamel- 

 forming tissue into one which simply fills up a void. 



It need hardly be added that a thin layer of enamel is developed 

 iipon the outside of the poison-fang ; but none is formed on the interior 

 of the poison-canal. 



The base of a poison-fang is fluted (this is not the case with other 

 Ophidian teeth), the dentine being convoluted as it is in the base of 

 the tooth of Varcmus, or in a labyrinthodont tooth ; and it is attached 

 to the bone through the medium of an opaque, ill-detiued, calcified 

 material, beyond which again comes a coarse bone. The fixation of a 

 tooth is effected (alike in cobra and in viperine suakes) by a sort of 

 scaftbldiug of coarse-textured bone, which is very rapidly thrown out 

 from the surface of its finer textural maxillary bone. This " bone of 

 attachment," met with, as I have elsewhere pointed out, in greater or 

 less quantity wherever teeth are attached by ankylosis, is entirely 

 removed with the fall of a tooth, and is developed afresh for its 

 successor. 



Nerve-supply to the Thyroid Gland. — In the last published part of 

 the volume for 1875, of Eobin's ' Journal de 1' Anatomic,' M. Poincare 

 states that he has been struck with the great richness of this gland in 

 nervous filaments of all sizes. This is the more curious, since the 

 gland presents no remarkable indications either of sensibility or 

 motility. No doubt the gland contains a large number of vessels, 

 which require, consequently, many vaso-motor fibres, but the nervous 

 supply is out of proportion to what may be supposed requisite for this 

 purpose, and M. Poincare thinks this peculiarity accounts in some 

 measure for the close relationship known to exist between the thyroid 

 gland and the generative organs, and believes that many of them are 

 of a sensory nature. The nerves form close plexuses surrounding 

 small islets of the substance of the gland, and the branches passing 

 to and from the gland stand (as he expresses it) in the relation of 



