I 



FUSIFORM CELLS. 157 



e intercellular substance, at the angles where three cells join each 

 other. The coalescence may be traced further than it is shown to do 

 in the figure ; so that if it were not for the evidence afforded by the 

 transition-stages here represented, it would be difficult to prove that the 

 membranous layer had its origin in cells. 



253. These facts, respecting the gradual coalescence of cells, explain 

 not merely certain appearances presented in Tooth, Shell, &c. (here- 

 after to be described) ; but also those which are exhibited by the Base- 

 ment-membrane, as already detailed ( 206). 



254. There is no evidence, in the preceding case, that the cavities of 

 the cells coalesce ; and there is no reason why they should do so. But 

 we often find such a union, where the production of a continuous tube 

 is required. The long straight open ducts, through which the sap of 

 Plants rises in the stem, are unquestionably formed by a coalescence of 

 the cavities of cells of a cylindrical form, placed regularly end to end ; 

 and it seems probable that the network of anastomosing vessels, through 

 which the elaborated sap finds its way to the various parts of the vege- 

 table fabric, is formed, in like manner, by the coalescence of cells, 

 arranged obliquely and transversely in regard to one another. In like 

 manner, the capillary Blood-vessels of Animals are usually believed to 

 originate in rows of cells, the cavities of which have run together by the 

 obliteration of the transverse partitions ; as the persistent nuclei of such 

 cells may be occasionally brought into view in the walls of the capilla- 

 ries. And the same appears to be the origin of the tubular fibres of 

 Muscular and Nervous tissue, which contain the elements characteristic 

 of those tissues ; these elements, the fibrilloe of muscle and the granular 

 pith of the nerve-tube, being evidently the secondary products of parent- 

 cells, which seem to remain as their investing tubuli, in the walls of 

 which the original nuclei are often to be seen ( 338 and 388). 



255. Besides these changes, the original cells may often undergo 

 marked alterations of form ; and this quite independently of any pressure 

 to which they may be subject. Thus the pigment-cells, as already men- 

 tioned ( 229), frequently exhibit a curious stellate form ; arising from 

 the development of radiating prolongations, which are put forth from 

 the original spheroid. A form which is frequently assumed by the cells 

 that are developed in fibrinous or plastic exudations, and w T hich is also 

 met with in the cells of tumours, both malignant (or Cancerous) and 

 non-malignant, is that which has received the designation of fusiform or 

 spindle-like, from its prolonged shape and pointed extremites. The 

 various stages of transition, which may be observed between the simple 

 rounded cell and the fusiform cell, have been shown in Fig. 7 ; and it is 

 there seen that, when the transformation has gone to its utmost extent, 

 the nucleus of the cell is no longer visible, so that it bears a close re- 

 semblance to a simple fibre. Such cells are found amongst the simple 

 fibrous tissues , and, in the opinion of many, they give origin to them. 

 The appearance of tissue composed of fusiform cells, is shown in Fig. 35 ; 

 this is seldom met with as a permanent part of the normal fabric ; but 

 it is a frequent product of morbid action. 



256. We now proceed with the description of the various tissues in 

 the Human body, which are composed of cells united or transformed 



