The Zoologist — July, 1873. 3589 



shape, as the general growth of the animal requires, that this last 

 process may take place : bone is occupied throughout its substance 

 by small hollow spaces, technically termed lacimee, which com- 

 municate with each other and with the nearest vascular surfoce by 

 means of very fine tubes termed canaliculi: these lacunse and 

 their canaliculi are occupied by soft living cells which seem to 

 possess the power of building up or taking down whatever is 

 required. 



Little animalcules (Foraminiferse by name) have the power of 

 secreting small shells around them, leaving fine holes all over the 

 shells through which to pass fine processes of their bodies, which 

 only consist of a little jelly-like protein. We may look upon each 

 of these cells occupying the lacunae of bone as so many Foramini- 

 ferae which have lost their individuality, and have had implanted in 

 them a sort of instinct, or habit, of building up around them, or 

 pulling down, or merely keeping in repair, just what is required by 

 the physiological well-being of the animal. Like a colony of bees, 

 always hard at work attending to their duty. In order to provide 

 them with requisite food, bone of any thickness is traversed by 

 vascular canals, called Haversian canals, which give fresh bone its 

 pink colour, and the blood-vessels within which, bring the food and 

 lake away the debris as required. Around these canals the cells 

 group themselves, communicating with them by the canaliculi. 



The problem to be solved in the construction of teeth is rather 

 different from that of bone. Here part of the organ has to resist 

 more or less severe direct mechanical friction, has to be exposed, 

 and at the same time maintain a strong connection with the living 

 and sensitive body. One of the first distinctions between tooth- 

 substance and bone seems to be in the elimination of the requisites 

 for pulling down and rebuilding. No normal tooth that I am aware 

 of alters its shape after formation. The calcigerous or bone-forming 

 cells retire to the circumference of the space around each vascular 

 canal, and dwindle in size until they disappear, or they retire into 

 the vascular canal and remain there as a persistent calcigerous 

 pulp. The fine canaliculi, around which the salts of lime which 

 harden the tooth were deposited, remain. Professor Owen men- 

 tions having observed the tooth of a fish composed only of this 

 structure, which he calls vaso-dentine ; an advance upon this vaso- 

 dentine is made by the whole exposed part of the tooth being 

 protected by a layer of the calcified tissue traversed by canaliculi, 



SECOND SERIES — VOL, YIII. 2 K 



