314 THE MUSCULATURE 



slum externum. The membranes, or muscle fasciae, are united to various parts 

 of the skeleton, either directly or by means of intermuscular septa, and, where 

 strong, keep the underlying musculature in place. In some regions they are united 

 to the muscles; in others they are separated from the underlying musculature by 

 loose areolar tissue, which allows free movement between the surface of the mus- 

 cles and the overlying fascia. The best example of a strong fascia of this nature 

 is that which envelopes the extensor muscles of the thigh. Where the fasciae are 

 well developed, the main bundles of constituent fibres take a course directly or 

 obliquely transverse to the direction of the unclertying muscles. They may be 

 composed of several successive laj- ers of fibrous tissue, the fibres of one layer tak- 

 ing a different direction from those of the next layer. 



The f miction of the fasciaj is the mechanical one of restraining or modifying muscle action. 

 The direction of the main component fibre-bundles indicates the direction of the greatest stress 

 to which the fasciae are subjected. Indirectly the fasciae pi'omote the circulation of the blood 

 and lymph in places where the vessels lie between the contracting muscles and the overlying 

 fascia. 



Intermuscular septa. — Muscle fasciae enclose not only the external layer 

 of the musculature of the body, "but also the various groups of more deepty 

 seated muscles. In addition, between the individual muscles, and between the 

 different layers and groups of muscles, there intervenes a greater or less amount of 

 connective tissue, sometimes loose in texture, sometimes dense in structure. In 

 these intermuscular septa run the chief nerves and blood-vessels of the region in 

 which the musculature lies. 



Gross structure of the muscles. — The muscles are composed of bundles of red- 

 dish fibres surrounded by a greater or less extent of white and glistening connective 

 tissue. They are attached by prolongations of this tissue in the form of tendons 

 or aponeuroses usually to the bony skeleton, but also in places to cartilages, as in 

 the thorax and larynx; to the skin, as in the face; to mucous membranes, as in the 

 tongue and cheeks; to the tendons of other muscles, as in the case of the lumbrical 

 muscles; to muscle fasciae, as in the case of the oblique and transverse muscles of 

 the abdomen; and to other structures, as, for instance, to the eyeball. 



The fleshy portion of the muscle is called the belly. The belly is usually 

 attached at one extremity to a portion of the skeleton or to some other structure 

 which serves as a support for its action on the structures to which its other ex- 

 tremity is attached. The attachment to the more fixed part is called the origin of 

 the muscle; the attachment to the structure chiefly acted on is called the insertion. 

 Thus the origin of the biceps muscle, the chief flexor of the forearm at the elbow, 

 is from the scapula; the insertion is into the radius and into the fascia of the fore- 

 arm. The part of the muscle attached to the origin is called the head of the mus- 

 cle. The part attached to the insertion is sometimes called the tail, but this term 

 is much less frequently used than the former. 



The muscles vary greatly in size and form. Thus the stapedius muscle of the 

 middle ear is a slender little structure, only a few millimetres long, while the glu- 

 teus maximus muscle of the hip is a large, rhomboid structure often several centi- 

 metres thick and with a surface area of over 500 square centimetres. The length 

 of a muscle from origin to insertion may be much less than the width of the muscle, 

 as in the intercostal muscles; or much greater than the width, as in most of the 

 long muscles of the limbs. The thickness of a muscle is usually less than the width 

 —so much so in some instances that the muscle is described as flat, sheet-like, or 

 ribbon-like: while in other instances the belly is cylindrical. In flat muscles the 

 general outline is usually quadrilateral or triangular. In triangular muscles in 

 most instances one angle of the triangle marks tlie insertion of the muscle, while 

 the opposite side marks the origin. In cylindrical muscles the belly usually has a 

 somcwiiat fusiform shape, and grows smaller both toward tlie origin and the in- 

 sertion of the muscle. 



Some muscles are divided by tendons transverse to the long axis of the muscle. 

 When one such tendon exists, the muscle is called digastric (fig. 348); when sev- 

 eral, polygastric, e. g., rectus abdominis (fig. 388). 



Tvv(j muscle masses with scpar;it(> origins may have a common insertion. Such 

 muscles arc usually designated bicipital muscles (biceps muscles of the arm and 

 thigh). Other muscles have three heads (the triceps muscle of the arm) or four 

 (the quadriceps muscle of the thigli). In the latter case special names are given 



