STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 45 



doubly refracting. These are ranged alternately across the fibre, 

 making the transverse markings or striae from which it gets its 

 name. This striated material is quite soft, and is incased in a thin 

 homogeneous elastic sheath called sarcolemma, which keeps the 

 fibre in its normal shape. 



This form of muscle may be considered the widest departure 

 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 

 fascia?. Muscles made of striated tissue are commonly 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 issue 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 



