172 THE ANIMAL CELL. 



Every texture taken as a whole was viewed by Bichat (Anatomic generate, 1801) 

 as constituting a peculiar system, presenting throughout its whole extent in the body 

 characters either the same, or modified only so far as its local connections and uses 

 render necessary ; he accordingly used the term " organic systems " to designate the 

 textures taken in this point of view. Of the organic systems above enumerated some 

 are found in nearly every organ ; such is the case with the connective tissue, which 

 serves as a binding material to hold together the other tissues which go to form an 

 organ ; the vessels, which convey fluids for the nutrition of the other textures, and 

 the nerves, which establish a mutual dependence among different organs. These 

 were named by Bichat the " general systems," to distinguish them from others such 

 as the cartilaginous and osseous, which being confined to a limited number or to a 

 particular class of organs, he named " particular systems." 



Structural elements. When any tissue is separated by the aid of the micro- 

 scope into the simplest parts which possess assignable form, such parts are termed 

 the structural elements of the tissue. In almost every tissue, some at least of these 

 structural elements retain to a certain extent the characters of the elementary 

 corpuscles of which the whole body is originally composed. These structural 

 elements are named the " cells " of the tissue. Others again lose for the most part 

 those characters, and becoming elongated and modified in structure are termed the 

 fibres of the tissue, whilst in other cases, fibres are formed not from the cells but 

 between them. Except the epithelium, all the tissues have fibres as characteristic 

 structural elements, and some, as the connective tissue, fibres of more than one 

 kind. But structurally and chemically as well as functionally, the fibres of the several 

 tissues differ widely from one another. 



Intercellular substance. In addition to these separable structural elements, 

 many of the tissues are composed of a homogeneous matrix or ground substance, in 

 which the structural elements are imbedded. This matrix may exist in considerable 

 quantity, as in some varieties of connective tissue, or on the other hand it may be 

 almost imperceptible in amount, serving merely as a cementing material to connect 

 together the individual tissue-elements, as in epithelium. From its softness, clear- 

 ness and homogeneity, this ground substance is often apt to escape observation, but 

 its existence may always be rendered evident in consequence of the property it 

 exhibits of combining with salts of silver, a brown deposit of metallic silver occurring 

 in it on subsequent exposure to the light (v. Eecklinghausen). 



Since all the animal tissues however diversified they may appear, originate as 

 collections of the elementary corpuscles or cells above spoken of, and since these cells 

 remain, many of them, as constituent elements of the formed tissue, we must first of 

 all consider minutely what it is that constitutes an animal cell, what is its structure, 

 its chemical composition, its physical and vital properties, and how it becomes 

 reproduced and multiplied. 



THE ANIMAL CELL. 



An animal cell is a corpuscle of microscopic dimensions, the cells of the human 

 body seldom exceeding -g^th of an inch in diameter, and many being as small as one- 

 tenth of this or even less. But whether small or large, every cell consists of two dis- 

 tinct parts : of the main substance of the cell, which has received the name of proto- 

 plasm (fig. 201,^?), and of a minute vesicular structure, generally placed near the 

 centre of the cell, and termed its nucleus (n). 



THE PROTOPLASM OF THE CELL. 



Structure of the cell-protoplasm. Until comparatively recent years it was 

 universally held that the principal or living substance of the cell to which the name 



