The Muscular System 117 



Cardiac Muscle (Fig. 34, A). — In many respects, cardiac muscle 

 resembles skeletal muscle : the individual fibers have cross striations 

 and are multinucleate. In other respects, however, they are quite 

 different. The individual fibers are branched and the whole heart 

 musculature is composed of a network of these branched and anastomos- 

 ing (connecting) strands with nuclei scattered at intervals. These 

 nuclei are located within the strands, not at the surface. This complex 

 network is separated into smaller units by conspicuous cross bands 

 known as intercalated discs. 



Cardiac muscle is found only in the heart and at the bases of the 

 large arteries that arise from it. This type of muscle has one unique 

 characteristic : even isolated bits will contract rhythmically. This 

 rhythmic contractibility thus appears to be an inherent property of 

 cardiac musculature. This tissue, like smooth muscle, is not under 

 voluntary control. 



THE MUSCLES 



A large muscle such as the one found in the calf of the leg has 

 three main parts : the main mass is the belly; the two points of at- 

 tachment are the origin and the insertion (Fig. 35). The origin is al- 

 ways on some point that does not move during the contraction of the 

 muscle; the insertion is on the part that is moved during contraction. 

 Most skeletal muscles have their origins and insertions on bones ; but oc- 

 casionally certain muscles are attached to the skin, connective tissues, 

 or even to other muscles. 



Most muscles that have their origin or insertion on bones are 

 fastened to the bones by means of tendons. These tendons are heavy 

 bands of fibrous connective tissue which are continuous with the pro- 

 tective fascia of the muscle. At their junction with the bones, the 

 tendons merge with the periosteum to form a very strong attachment. 

 In some instances, the fascia does not form tendons, but continues as 

 a broad sheet to the point of attachment. 



Levers.— Movements brought about by the muscles depend upon 

 their attachments and the nature of the skeletal joint involved. In 

 many of these movements, the physical advantages of various types of 

 levers are used. 



There are three principal classes of levers. Those of the first 

 class have the fulcrum in the center with the weight to be moved at one 

 end and the power at the other. A seesaw is a good example of this 



