MAKBIBITLAE Al^D PALATAL TEETH OE CTE^'ODUS. 118 



The base or plate Avlien cut vertically and examined under 

 moderate powers, presents large canals traversing the bony sub- 

 stance, and which gives oiF branches at all sorts of angles. These 

 branches vary slightly in length, but they are always short and 

 soon anastomose with each other. The irregular branching and 

 ready inosculation, combined with the fact that the diameters of 

 the branches are quite as great as those of the main vessels from 

 which they arise, cause the base to appear riddled with an 

 irregular open network of vessels ; nor does this character alter 

 when the base is taken horizontally, thus showing that the canals 

 at this point have not any particular course. The proportion of 

 bone tissue to canals is about equal. The canals in this part of 

 the tooth do not give off any ramuscles to penetrate the osseous 

 substance. The bone, however, is supplied with nutriment by 

 means of lacunae. These lacunae, or bone-cells, are characteristic 

 of true bone ; but in this tissue they differ from those of true 

 bone in their form and arrangement. They are numerous, and are 

 not arranged in concentric circles round the canals, but are dis- 

 persed throughout the bony tissue in an irregular fashion. In size 

 the bone- cells are large, and present more of a reptilian character 

 than of a piscine. In a vertical section the lacunae have an 

 exceedingly elongated form, the long diameter being frequently 

 parallel with the course of the medullary canals they accompany ; 

 but often they lie without any such order. When cut transversely 

 through the centre the cells assume an almost circular form. The 

 bone-cells vary somewhat in length and breadth. All the lacunae 

 give off canaliculi, which ramify in the osseous substance, and 

 inosculate with canaliculi springing from neighbouring lacunae 

 or else empty themselves into the adjoining canal. In order to 

 examine these lacunae with their canaliculi, it is not necessary to 

 use high powers on account of their size, but they are often 

 absent in sections, this being due to the cuttings having been made 

 too thin, and the lacunae ground away altogether. The bone 

 tissue is homogeneous, no structure being observable under any 

 power of the microscope. 



As we follow the network of medullary canals to the upper 

 surface of the plate we find that it gradually assumes a more 

 regular appearance from the canals becoming smaller in diameter 

 and pursuing a more vertical course near the junction of the 

 plate with the ridges ; the network slowly changes and becomes 

 obliterated. This alteration is caused by the branches that are 

 given off by the main canals arising at a more and more acute 

 angle, and pursuing a more and more vertical course, thus 

 necessitating anastomoses of the branches with each other at 

 higher points than their places of origin — the branches still pre- 

 sent a similar diameter to the parent canals. The bone tissue at 

 the same time becomes more and more out of proportion to the 



