STRUCTURAL ( 'II A RACTKRS OF ANIMAL ORGANISMS. 53 



from the primitive protoplasmic type imparting to it contractility. 

 It moves with wonderful rapidity, contracting almost the instant 

 its nerve is stimulated. It forms the great mass of the quick- 

 acting skeletal muscles, being attached to the bones by bands 

 composed of a form of fibrous connective tissue, which form the 

 tendons and fasciae. Muscles made of striated tissue are com- 

 monly under the control of the will, and hence are frequently 

 spoken of as voluntary muscles, but this term is misleading, for 

 many striated muscles are not governed by voluntary control. 



The Connective Tissue group, coming exclusively from 

 the middle germinal layer, exhibits very great varieties of form. 

 Its cells differ much from the epithelial cells both in their char- 

 acter and their relations, and particularly in the adult tissues. 



Under the heading Connective Tissues are generally classed all 

 those which support the frame and hold together the various 

 other tissues and organs. They are 



1. Mucous and retiform connective tissue. 



2. White and yellow fibrous tissue. 



3. Cartilage. 



4. Bone as well as certain modifications of these types. 

 The cells of all these tissues have the property of manufacturing 



some material, which, however, does not generally inclose them as 

 a cell wall, but remains between the cells and forms the intercel- 

 lular substance. The younger the tissue the greater is the pro- 

 portion of its cell constituents, and the older the tissue the greater 

 will be found the preponderance of the intercellular substance. 



MUCOUS Tissue. In certain parts of the embryo and in some 

 of the lower animals a kind of connective tissue is found in which 

 there is but little intercellular substance, the mass of the tissue 

 being thus made up of cells. This cellular connective tissue never 

 forms an important texture in the adult man, but is interesting 

 as the probable tissue from which all the connective tissues are 

 formed in the embryo, and as occurring in abnormal growths or 

 tumors. 



The first step in differentiation is the secretion of a large quan- 

 tity of soft, homogeneous, semi-gelatinous or fluid material like the 

 mucus secreted by epithelium. In this the cells lie, either free 



