ANATOMY OF THE HOBSB. 277 



known as the voluntary muscles, appear in the form of fleshy 

 structures, red in color, and with fibers of various degrees of fineness, 

 and are composed of fasciculi, or bundles of fibers, united by con- 

 nective or cellular tissue, each fasciculus being composed of smaller 

 ones, less in size, but united in a similar manner to compose the 

 larger formations, each of which is enveloped by a structure of similar 

 nature known as the sarcolenvma. Many of the muscles are united to 

 the bones by the direct contact of their fleshy fibers, but, in other 

 instances, the body of the muscle is more or less gradually trans- 

 formed into a cordy or membranous structure known as the tendon 

 or sinew, and the attachment is made by very short fibrous threads 

 through the meditim of a long tendinous band, which, passing from 

 a single one to several other of the bones, effects its object at a point 

 far distant from its original attachment. In thus carrying its action 

 from one bone to another, or from one region of a limb to another, 

 these tendons must necessarily have smooth surfaces over which to 

 glide, either upon the bones themselves or formed at their articula- 

 tions, and this need is supplied by the secretion of the synovial fluid, 

 a yellowish, unctuous substance, furnished by a peculiar tendinous 

 synovial sac designed for the purpose. 



Illustrations in point of the agency of the synovial fluid in assisting 

 the sliding movements of the tendons may be found under their vari- 

 ous forms at the shoulder joint, at the upper part of the bone of the 

 arm, at the posterior part of the knee joint, and also at the fetlocks, 

 on their posterior part. 



As the tendons, whether singly or in company with others, pass 

 over these natural pulleys they are retained in place by strong fibrous 

 bands or sheaths, which are by no means exempt from danger of 

 injury, as will be readily inferred from a consideration of their im- 

 portant special use as supports and reenforcements of the tendons 

 themselves, with which they must necessarily share the stress of what- 

 ever force or strain is brought to bear upon both or either. 



We have referred to that special formation of the external surface 

 of a bone by which it is adapted to form a joint or articulation, 

 either movable or fixed, and a concise examination of the formation 

 and structure of the movable articulations will here be in place. 

 These- are formed generally by the extremities of the long bones, or 

 may exist on the surfaces of the short ones. The points or regions 

 where the contact occurs is denominated the articular surface, which 

 assumes from this circumstance a considerable variety of aspect and 

 form, being in one case comparatively flat and "another elevated ; or 

 as forming a protruding head or knob, with a distinct convexity ; and 

 again presenting a corresponding depression or cavity, accurately 

 adapted to complete, by their coaptation, the ball and socket joint. 

 The articulation of the arm and shoulder is an example of the first 



