i GENERAL CONSIDEBATIONS. 



bodies, as on the surface of the skin ; at other times they become dissolved, 

 as in some glands ; finally, at other periods they submit to fatty degeneration, 

 which gradually brings about their complete destruction. 



The permanent cells have been arranged according to the following 

 denominations : — 



1. Hcemaiies or red globules, which are found in a state of suspension 



in the blood ; they are round or elliptical. 



2. Leucocytes, or white globules, which float in the blood, lymph, and chyle. 



3. Coimective cells, comprising the connective cell properly so-called, 



the plasmatic cell, and the adipose cell. 



4. Medullary cells, forming the principal elements of the marrow of 



bones (myeloplaxes and medullo-cells). 



5. Contractile cells, which constitute the basis of muscular tissue. 



6. Nerve cells, met with in the cerebro-spinal centres and the ganglia 



of the cerebro-spinal and sympathetic systems. 



7. Epithelial cells, comprising the epithelial cells properly called, 



situated on the surface of the skin and mucous membranes, and 

 the glandular cells. 



Fibres. — A fibre is an elongated anatomical element, of variable dimensions 

 and composition. It may be very fine and represented by a single line, or 

 thicker and marked by two lines more or less apart from one another. It 

 is homogeneous throughout, or the contents are distinct from the envelope. 



The vitality of fibres is not to be compared with that of cells ; after they 

 are formed, they can only be nourished, and cannot multiply of themselves. 

 In the animal economy four kinds of fibres are distinguished : — the connective 

 fibre, elastic fibre, muscular fibre, and nervous (or nerve) fibre. 



Tissues. — The elements that have now been rapidly described, in 

 becoming united and grouped in difierent fashions, form the tissues. 



Some tissues are composed of one kind of element ; these are the simple 

 tissues. Example : — The epithelium. 



• The majority, however, are formed by the union of several different 

 elements ; these are the composite tissues. Example : — Nervous tissue. 



It is also remarked that there are tissues in which exists a fundamental 

 intercellular substance, and others in which this is absent. The latter are 

 few in number, for the vessels and nerves may, in certain tissues, be 

 considered as intercellular substance. 



The anatomical, physico-chemical, and physiological characters of the 

 tissues repeat, as might easily be inferred, the anatomical, physico-chemical, 

 and physiological properties of the elements entering into their formation. 



Only four fundamental tissues are recognised, basing them on the 

 morphological, chemical, and physiological characters of the elements. 



In the first place, the tissue of the conjunctival substance should be noticed ; 



this, in consequence of some differential characters, may be divided into : 



gelatinous tissue, conjunctival (or connective) tissue, cartilaginous tissue, and bony 



Then comes the cellular tissue, formed entirely of persistent cells. It 

 comprises the epithelial tissue and the glandular tissue. 



The cells of the epithelial tissue may affect different arrangements. If 

 they are disposed in a single row, there results a simple epithelium ; if they 

 are superposed it is a stratified epithelium. According to the form of the 

 cells of the superficial layer, the eTpitheiinm is polyhedral, pavement, cylindrical, 

 01 spherical. In certain points, these superficial cells are furnished with 

 vibratile filaments ; they are then designated vibratile (or ciliated) epithelium. 



