350 MICROSCOPIC ANATOMY OF THE TEETH 



look upon this epithelial layer which he describes as con- 

 sisting of the cells of the deeper part of the follicle. 



Nasmyth's membrane not being shown would suggest the 

 probability that the separating layers forming this membrane, 

 as shown in figs. 230 and 231, &c., have become completely 

 detached and lost in the preparations figured in the paper. 

 In several of the author's sections this has occurred, and 

 would be probably still more likely to happen with paraffin 

 sections treated with alcohol. 



In this case, as suggested, it is the deeper epithelial cells 

 of the follicle which are connected with the surface epithelium 

 and not the cells of the enamel organ. 



As it is considered by Fischer that the outer and inner 

 layers of the enamel organ become united and blended with 

 the surface epithelium, he cannot apparently agree with the 

 views of Von Brunn and Ballowitz that these layers are 

 continued downwards as the sheath of Hertwig, and in fact, 

 in his figures, the sheath of Hertwig is shown as discon- 

 tinuous at the neck of the tooth and is neither a prolongation 

 of the cells of the enamel organ nor of the deeper epithelial 

 cells of the follicle. 



Teeth do not always erupt in a vertical direction as in man, 

 where the new tooth appears immediately beneath that 

 which is being shed. 



In osseous fish the successional teeth usually appear at 

 the sides of the tooth in use, but in Sargus both the molars 

 and incisors erupt directly beneath the tooth which is shed, 

 and in the pharyngeal teeth of Labrus the same mode of 

 succession is seen. In the Sharks new teeth come into use 

 in successive rows, being carried forward by the movement 

 of the mucous membrane over the rounded cartilaginous 

 jaws. 



Some teeth never erupt, but remain embedded in the jaws, 

 as in the female Narwhal, and many instances are recorded 

 in man where teeth have never erupted or have appeared 

 long after the normal period of eruption. 



Although we have some knowledge of the phenomena 

 accompanying the process, the actual conditions necessary 

 and the forces which govern the process of the eruption of 

 the teeth are still but very imperfectly understood. 



