MYOLOGY 253 



dense white fibrous tissue by means of which a muscle is attached; an aponeurosis 

 is a broad fibrous sheet which fulfils a similar function. 



(4) The action belongs rather to physiological study, but the main points are 

 usually given in anatomical descriptions. In some ciises the action is simple, in 

 others, complex. INIuscles which concur in action are termed synergists; those 

 which have opposite actions are antagonists. 



(5) The consideration of structure includes the direction of the muscular fibers, 

 the arrangement of the tendons, the synovial membranes, and any other accessory 

 structures. The terms fleshy and tendinous are sometimes used to indicate the 

 relative amounts of muscular and tendinous tissue. In the case of the long muscles 

 of the limbs, the origin is termed the head (Caput), and when the muscle is fusi- 

 form, the large fleshy part is often called the belly (^'cnter) of the muscle. S(jme 

 muscles have two or more heads, and are hence designated as biceps, triceps, etc. A 

 digastric muscle is one having two bellies and an intermediate tendon. In most 

 muscles the muscle-fibers join the tendon at an acute angle, like the relation of the 

 barbs of a feather to its shaft; hence the term pennate is applied to such an arrange- 

 ment. When the fibers are so arranged on one side of the tendon the muscle is 

 xinipennate (^I. unipennatus) ; while one in which this arrangement exists on l)oth 

 sides is bipennate {M. bipennatus). The structure may be still more complex, re- 

 sulting in a multipeimate muscle. The structure of many muscles is much more 

 complex than a superficial examination would lead one to suppose. Frequently 



Tilroas sheath J^lesatenclolj Fihrous sheatk, 



^"■^ '"■ ' ' A '" B 



Fig. 260. — Di.vgkams of Cross-sections of Synovial Btjbsa (A) and Synovial Sheath (B) ; T, Tendon. 

 In both the synovial sac is represented for the sake of clearness as though somewhat distended. 



they are intersected by tendinous layers or bands, knowTi as tendinous intersec- 

 tions. Intersecting bands or tracts which appear on the surface — usually as zig- 

 zag fines — are termed tendinous inscriptions (Inscriptiones tendinese). 



(6) The relations constitute a very important part of anatomical topography, 

 and a knowledge of them is fundamental to further study in this respect. 



(7) The blood and nerve supply are, of course, important on clinical grounds. 

 The nerve supply is often of value in the determination of homologies. As might 

 be expected, the muscles have a large blood supply. They are also provided wth 

 lymph-vessels. The nerves to the muscles are motor, sensory, and vasomotor in 

 function. 



The accessory structures associated with the muscles are the sATiovial mem- 

 branes and the fasciae. 



The synovial membranes are thin-walled sacs, similar to the synovial mem- 

 branes of the joints, and having a similar function. Two forms are recognized. 

 A synovial bursa (Bursa synovialis) is a simple sac which is interposed at a pomt of 

 unusual pressure between a tendon or muscle and some underlying structure, com- 

 monly a prominence of the skeleton. A synovial sheath (Vagina synovialis ten- 

 dinis) differs from a bursa in the fact that the sac is folded around the tendon so that 

 two layers can be distinguished: the inner one is adherent to the tendon, while the 

 outer one lines the canal in which the tendon lies. The two layers are continuous 

 along a fold termed the mesotendon. The arrangement is shown in the annexed 

 diagrams. The synovial membranes of joints in some places form extra-articular 



