SECEETING CELLS. 83 



to the dentinal tissue, as has been so long taught ? 

 There are no tubes fulfiling a similar office in ivory 

 a tissue very analogous to the dentine. Surely, if 

 the ivory of the elephant's tusk can be formed and 

 preserved in a healthy state without nutrient fluids 

 being conveyed by tubes to every part of it, it is 

 extremely probable that the dentine of other mam- 

 malian teeth is in like manner destitute of any such 

 special provision, as has been conjectured, for its free 

 irrigation in every part. These tissues really undergo 

 little change after their formation, and such an ex- 

 tensive system of nutrient channels as has been sup- 

 posed to exist would be perfectly useless. 



The formation of the soft tissue of dentine and 

 enamel affords an interesting example of the growth 

 movement in opposite directions of masses of bioplasm 

 destined to produce special structure. In each case 

 the bioplasts move towards a vascular tissue which 

 recedes as they advance, and which wastes when the 

 formation of the tissues and their calcification have 

 been completed. 



124. Of Secreting Cells. Contrasting in most 

 important particulars with the epithelial cells of the 

 mouth, already referred to in 33, are the " cells " 

 which are concerned in the formation of secretions, 

 of which the liver cell may be taken as an example. 

 This elemental unit consists of a spherical mass of 

 bioplasm, often containing new centres of growth 

 (nuclei), surrounded by a considerable extent of soft 

 formed material, giving to the whole an irregularly 

 oval or somewhat angular appearance. . Sometimes 

 there are two or even three masses of bioplasm in one 

 "cell," in which case the mass looks more like a 

 portion of a cylinder than a " cell." The formed 

 material is undergoing change upon its outer surface, 

 and, although resulting from changes in one kind of 

 bioplasm, becomes gradually resolved into sugar and 



G 2 



