DR. BEALEj ON THE TISSUES. 237 



in the early life of every creature they are distinct enough in 

 every tissue. In the higher animals these elementary parts 

 are arranged in certain collections which possess very differ- 

 ent endowments. 



In some of the simplest living beings the entire organism 

 may be regarded as consisting of one elementary part. 



Every elementary part comes from a pre-existing elemen- 

 tary part ; but it does not follow that its endowments are to 

 be the same as those of the elementary part from which 

 it sprung. 



We must not look upon the elementaiy parts of a tissue as 

 bodies which, having assumed a definite form and reached a 

 certain size, remain perfectly stationary, but as structures 

 which are continually undergoing change — not a single par- 

 ticle of which they are composed is still. It is true the move- 

 ments occur so slowly in some as to be imperceptible, except 

 after long intervals of time, while we can scarcely conceive 

 the rapidity with which change takes place in others. But 

 movements must occur in all, and they take place in the same 

 direction. The elementary parts, which "^e examine in our 

 microscopes, were undergoing change just before they were 

 removed from the living structure. We have stopped the 

 changes at a certain point, and, as the ages of the elementary 

 parts differ materially, by carefully comparing the appearances 

 in several, we may obtain, after numerous observations, data 

 which enable us to form something like a connected history 

 of the life of one of them. 



The cell-wall not a constant or essential structure. — These 

 elementary parts are usually termed cells, and the cell is de- 

 fined as an organ, consisting of a ivall permeable to fluids, 

 with certain contents within, and usually, but not constantly, 

 a nucleus. In the process of secretion it is believed that 

 certain materials pass through this wall into the interior of 

 the cell by endosmose, and then become altered by powers 

 existing in the cell or resident in the nucleus, and, having 

 undergone conversion into new substances, pass through the 

 wall of the cell by exosmose, and constitute the special 

 secretion. In tissues it is believed that the cell exerts a 

 peculiar action on the matter which surrounds it, by reason 

 of which this manifests certain peculiar and characteristic 

 properties. 



It is the exception rather than the rule to find that the con- 

 tents of a cell are in a fluid state, and when this is so, numsrous 

 living particles are generally suspended in it. In the liver- 

 cell the contents are certainly tolerably firm. In the kidney- 

 cell they present the same characters. Their consistency 



