CELLS. 31 



ing solid, and uniting end to end for greater length, or by lat- 

 eral branches to form a network. So, according to Henle, are 

 formed the filaments of the striated and fenestrated coats of 

 arteries ; and according to Beale, the so-called connective-tis- 

 sue corpuscles are to be considered branched nuclei, formed 

 of protoplasm or germinal matter. 



3. Cells. The word " cell " of course implies strictly a hollow 

 body, and the term was a sufficiently good one when all so- 

 called cells were considered to be small bags with a membra- 

 nous envelope, and more or less liquid contents. Many bodies, 

 however, which are still called cells do not answer to this de- 

 scription, and the term, therefore, if taken in its literal signifi- 

 cation, is very apt to lead astray, and, indeed, very frequently 

 does so. It is too widely used, however, to be given up, at 

 least for the present, and we must therefore consider the term 

 to indicate, either a membranous closed, bag with more or less 

 liquid contents, and almost always a nucleus ; or a small semi- 

 solid mass of protoplasm, with no more definite boundary-wall 

 than such as has been formed by a condensation of its outer 

 layers, but with, most commonly, a small granular substance 

 in the centre, called, as in the first place, a nucleus. In both 

 cases the nucleus may contain a nucleolus. Fat-cells (Fig. 11) 

 are examples of the first kind of cells ; white blood-corpuscles 

 (Fig. 26) of the second. 



The cell-wall, when there is one, never presents any appear- 

 ance of structure : it appears sometimes to be an albuminous 

 substance ; sometimes a horny matter, as in thick and dried 

 cuticle. In almost all cases (the dry cells of horny tissue, 

 perhaps, alone excepted) the cell-wall is made transparent by 

 acetic acid, which also penetrates into the interior and distends 

 it, so that it can hardly be discerned. But in such cases the 

 cell-wall is usually not dissolved ; it may be brought into 

 view again by merely neutralizing the acid with soda or 

 potash. 



The simplest shape of cells, and that which is probably the 

 normal shape of the primary cell, is oval or spheroidal, as in 

 cartilage-cells and lymph-corpuscles ; but in many instances 

 they are flattened and discoid, as in the red blood-corpuscles 

 (Fig. 26) ; or scale-like, as in the epidermis and tessellated 

 epithelium (Fig. 2). By mutual pressure they may become 

 many-sided, as are most of the pigment-cells of the choroidal 

 pigmentum nigrum (Fig. 12), and those in close-textured 

 adipose tissue ; they may assume a conical or cylindriform or 

 prismatic shape, as in the varieties of cylinder-epithelium 

 (Fig. 4) ; or be caudate, as in certain bodies in the spleen ; 

 they may send out exceedingly fine processes in the form of 



