SUCCESSION OF THE POISON-FANGS OF SNAKES. 
379 
the order in which they would succeed to the tooth already in use. It will be seen that 
the new tooth comes alternately from either side, No. 2 being on the left- and No. 3 on 
the right-hand side in fig. 2. There is an obvious advantage in such an arrangement, 
since it insures the least possible loss of time between the shedding-off of one poison- 
fang and its replacement by another ; but if the new tooth passed exactly into the posi- 
tion of its predecessor little or nothing would be gained. Such is not the case, however ; 
there is room upon the surface of the maxillary bone which gives attachment to the 
poison-tooth for two teeth, side by side (see fig. 4). The tooth actually in use occupies 
one side, where it is anchylosed by its base, side, and front, whilst its successor is getting 
ready to take possession of the vacant place by its side. Thus one poison-fang occupies 
the innermost position possible on the maxillary bone, the next one the outermost, and 
so they go on alternating. It will at once be obvious that such an arrangement of the 
successional poison-fangs is admirably adapted for securing a speedy replacement of a 
lost tooth ; and sections taken near to the base of attachment almost invariably (unless 
it so happens that the attached tooth has only just become fixed and its predecessor but 
just shed off) show a second tooth (2 in fig. 4) which is in the process of attachment by 
the rapid development of a coarse bone (b in the figure) about its base. 
In fig. 2 ( 1 ) the tooth in place alone occupies the chamber in the pouch of mucous 
membrane which covers in the recumbent poison-fangs, all the successional teeth still 
remaining bedded in the mass of loose connective tissue in the midst of which they 
are developed. 
In fig. 1, however, a different stage in the process is represented ; it is taken from a 
section of the head of an English viper, in which I was so fortunate as to hit upon the 
moment, so to speak, of the change from an old tooth to a new one. The chamber in 
the mucous membrane is seen to contain sections of two teeth, which are separated from 
one another by a flap of connective tissue ( b ). This flap of connective tissue, visible 
also in fig. 2, is continuous with the slightly specialized connective tissue which lies 
between and separates the two series of successional teeth ; and an apparent use for it is 
very obvious, though only actual observation can determine whether it has any other. 
It would seem probable that it serves to keep the advancing tooth to its own side, as it 
would be almost impossible for a tooth of the one series to get round it and so become 
wrongly placed on the side appropriated to the other series ; it would also serve, by 
limiting the space in which the advancing tooth lies, to keep it straight— that is to say, 
to prevent it from diverging to the one side or the other in its own length. 
In fig. 1 the tooth already used. ( 1 ) is about to be cast off, and its successor ( 2 ) has 
already passed out of the region where teeth are being developed, and is doubtless 
becoming rapidly attached. In fact fig. 4 is a section from the very same specimen, 
though, unfortunately for the comparison, the section has been mounted with the other 
surface uppermost, so that the sides are transposed. However, 2 on the left of fig. 4 is 
the very same tooth as 2 on the right of fig. 1, the first-named section being taken close 
3 G 2 
