178 STRUCTURE OF THE VERTEBRATES 



aponeurosis, or sheet of connective tissue. Similar changes in 

 function are found in numerous other muscles. The important 

 thing to understand is the usual or major function of a muscle 

 or group of muscles. 



The tendons and ligaments which attach the muscles to other 

 structures (a bone or other muscle) are important in their func- 

 tion. All muscle fibers are attached by connective tissue strands 

 to the periosteum of the bone or to other connective tissues. As 

 a long muscle approaches its point of insertion the amount of 

 connective tissue increases as the number of fibers decreases; 

 until near its insertion only a band or cord of tendinous tissue 

 is present. The fibers of the tendon become interwoven with 

 those of the periosteum, in that way firmly attaching the muscle. 



During the course of evolution the tendency has been for 

 muscle groups to divide into smaller units, thus increasing the 

 total number of separate muscles. Consequently, several muscles 

 may be inseparable at the point of origin. In tracing the his- 

 tory of a muscle the progressive stages of the splitting can be 

 found, varying from a single muscle which inserts by two or 

 more tendons to those which are separate throughout their en- 

 tire length. What appears to be the opposite condition is also 

 found, a single muscle arising from several heads, or points of 

 origin. 



Although the muscles furnish the power in any movement, 

 the skeleton gives form and rigidity to the body, flexibility 

 being gained by the joints between the individual bones. The 

 contraction of muscle results in various movements of the bones, 

 acting upon them as any power acts upon a lever. A number 

 of motions are possible, their range being limited by the struc- 

 ture of the joints and the articulations between bones. The more 

 important skeletal movements follow. 



1. Flexion is the formation of an angle between two bones. 

 When the arm is bent at the elbow it is flexion between the 

 upper and lower arm. Similarly the hand may be flexed in 

 relation to the forearm, or the fingers flexed toward the palm. 



2. Extension refers to straightening an angle already formed. 

 After a part is flexed the extensors pull it back into the original 

 position. 



