FRAGILITY OF THE TAIL IN LIZARDS 105 



septum in the middle of the caudal vertebral centra, 

 whereby these vertebrae are enabled to fracture trans- 

 versely at the position of the septum. The object of 

 this chapter is to inquire into the truth or otherwise 

 of this view, and to point out how this explanation 

 is insufficient to account for all the facts. Further, if 

 possible, to show that other factors play a part in the 

 process, without which it could not occur. 



Theoeetical Considerations. 



It had long appeared to me that the views enun- 

 ciated above, and copied into all text-books, even if 

 true as far as they went, afforded but a partial ex- 

 planation of the possibility of a lizard to lose a portion 

 of its tail with ease. The mere fact that an animal 

 has sustained a transverse fracture of a bone in a limb 

 or organ, is not enough to account for that limb be- 

 coming detached from the body. 



Bones are not the only parts of an organ, they are 

 kept in position by ligaments and by muscles acting 

 upon them, by connective tissue affording further sup- 

 port, and finally by the integument enclosing more or 

 less firmly the whole organ or limb. (An appendage, 

 such as the tail of a lizard, is practically an axial 

 limb.) The usual arrangement of these tissues other 

 than bones has the effect, when a fracture occurs, of 

 keeping the limb in connection with the body until a 

 more or less perfect union has taken place at the point 

 of fracture. In other words, for a limb to become 



