DESCBIPTION OP THE TISSUES. 483 



a piece of harness leather demands continual oiling to keep its 

 strength, but they undergo no change or alteration in their form 

 until destroyed by death. 



(c) Cells, which may even be regarded as low forms of life, are 

 masses of protoplasm or amorphous living matter, with a nucleus and 

 frequently a nucleolus, which are capable of assimilating nutriment or 

 food, propagating themselves either into others of the same form or 

 into fixed cells of another outward appearance and different function 

 but of the same constitution. It is simply in the mode of grouping 

 of these elements that we have the variation in tissues, as (1) loose 

 connective tissue, (2) aponeurosis and tendons, (3) muscles, (4) 

 cartilage, (5) bones, (6) epithelia and endothelia, (7) nerves. 



(1) Loose connective tissue forms the great framework, or scaf- 

 folding, of the body, and is found under the skin, between the muscles 

 surrounding the bones and blood vessels, and entering into the 

 structures of almost all of the organs. In this the fibers are loosely 

 meshed together like a sponge, leaving spaces in which the nutrient 

 fluid and cells are irregularly distributed. This tissue we find in the 

 skin, in the spaces between the organs of the body where fat accumu- 

 lates, and as the framework of all glands. 



(2) Aponeurosis and tendons are structures which serve for the 

 termination of muscles and for their contention, and for the attach- 

 ment of bones together. In these the fibers are more frequent and 

 dense, and are arranged with regularity, either crossing each other or 

 lying parallel, and here the cells are found in minimum quantity. 



(3) In the muscles the cells lie end to end, forming long fibers 

 which have the power of contraction, and the connective tissue is in 

 small quantity, serving the passive purpose of a band around the con- 

 tractile elements. 



(4) In cartilage a mass of firm amorphous substance, with no vas- 

 cularity and little vitality, forms the bed for the chondroplasts, or 

 cells of this tissue. 



(5) Bone differs from the above in having the amorphous matter 

 impregnated with lime salts, which gives it its rigidity and firmness. 



(6) Epithelia and endothelia, or the membranes which cover the 

 body and line all of its cavities and glands, are made up of single or 

 stratified and multiple layers of cells bound together by a glue of 

 amorphous substance and resting on a layer composed of fibers. 

 When the membrane serves for secreting or excreting purposes, as 

 in the salivary glands or the kidneys, it is usually simple; when it 

 serves the mechanical purpose of protecting a part, as over the tongue 

 or skin, it is invariably multiple and stratified, the surface wearing 

 away while new cells replace it from beneath. 



(7) In nerves, stellate cells are connected by their rays to each 

 other, or to fibers which conduct the nerve impressions, or they act as 



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