THE STRUCTURE OF THE M'OTOR ORGANS 83 



is found in connection with the lower jaw, It arises by a 

 tendon attached to the base of the skull ; from there its first belly 

 runs downwards and forwards to the neck by the side of the 

 hyoid bone, where it ends in a tendon which passes through a 

 loop serving as a pulley. This is succeeded by a second belly di- 

 rected upwards towards the chin, where it ends in a tendon in- 

 serted into the lower jaw. Running along the front of the abdo- 

 men from the pelvis to the chest is a long muscle on each side of the 

 middle line called the rectus abdominis: it is poly gastric, consisting 

 of four bellies separated by short tendons. Many uiuscles more- 

 over are not rounded but form wide flat masses, as for example the 

 muscle Ss seen on the ventral side of the shoulder-blade in Fig. 38. 



Gross Structure of a Muscle. However the form of the skeletal 

 muscles and the arrangement of their tendons may vary, the 

 essential structure of all is the same. Each consists of a proper 

 striated muscular tissue, which is its essential part, but which is 

 supported by connective tissue, nourished by blood-vessels, and 

 has its activity governed by nerves so that a great variety of 

 things go to form the complete organ. 



A loose sheath of areolar connective tissue, called the peri- 

 mysium, envelops each muscle, and from this partitions run 

 in and subdivide the belly into bundles or fasciculi which run 

 from tendon to tendon, or for the whole length of the muscle when 

 it has no tendons. The coarseness or firmness of butcher's meat 

 depends upon the size of these primary fasciculi, which differs in 

 different muscles of the same animal. These larger fasciculi are 

 subdivided by finer connective tissue membranes into smaller 

 ones, each of which consists of a certain number of microscopic 

 muscular fibers bound together by very fine connective tissue and 

 enveloped in a close network of blood-vessels. Where a muscle 

 tapers the fibers in the fasciculi become less numerous, and when a 

 tendon is formed disappear altogether, leaving little but the con- 

 nective tissue. 



Histology of Skeletal Muscle. Each muscle-fiber is developed 

 from a single cell and so constitutes a single histological element. 

 In the adult form, however, a muscle-fiber differs from an ordinary 

 cell in that it contains several nuclei. Muscle-fibers vary greatly 

 in size; ranging in length from 1 up to 35 mm. (^ in. to li in.), 

 and in diameter from 0.034 to 0.055 mm. (^^ to ^5^ in.). Each 



