38 HUMAN PHYSIOLOGY 



tion followed. Within a decade the scientific world had 

 fairly grasped the conception which we know as the Cell 

 Theory. What this is and what it implies must now be 

 shown with some care. What do we mean when we 

 speak of a cell? 



It must be admitted at once that the word cell is not 

 particularly appropriate in the sense in which it has 

 come to be used. A cell is properly a walled space, as 

 in the prison or the honeycomb. In biologic parlance 

 a cell is a small parcel of living matter. It is easy to 

 explain how the name began to be used. Dead and dry 

 vegetable substance, like cork which was one of the first 

 forms studied, has a structure which can be correctly 

 described as cellular. There are myriads of micro- 

 scopic spaces set apart by walls. The arrangement was 

 pictured carefully by Robert Hooke about the year 

 1665. The cells seen by this early worker were actual 

 cavities, but we have come to employ the term for the 

 separate packets of organized material which occupied 

 these cavities when the tissue was living and growing. 



The microscope shows that bits of animal matter also 

 consist of assembled units of about the same average 

 size as those of plants. But the so-called cells in the 

 animal are not commonly isolated by rigid and durable 

 partitions. Between neighboring ones the intervals may 

 be occupied by fluid, or partially taken up by fibers, or 

 there may be more or less compact deposits, but in 

 general one receives the impression from observation of 

 animal cells that they are often in practical contact 

 one with another and that it is curious that they do not 

 run together. There is reason to suppose that the 

 surface of each cell presents a film somewhat different in 

 character from the interior; this is sometimes spoken 

 of as the cell-membrane, but it is of extreme delicacy and 

 not to be compared with the substantial walls which 

 bound the cells of plants. 



Cells vary widely in size. There are probably more 

 which measure less than Kooo in<:h in diameter than there 



