GENERAL CONSIDERATION OF THE VOLUNTARY MUSCLES. 471 



there with the periosteum, and convert the grooves into canals within which the ten- 

 dons are enclosed, although capable of free movement to and fro. These connective- 

 tissue bands are the tendoyi-sheaths, and the canals which they assist in forming 

 may contain one or more tendons. Each sheath is lined on its deeper surface by a 

 synovial membrane similar to those occurring in the joints, and at either extremity 

 of the sheath this membrane is reflected ypon the tendon which it encloses, so that 

 the tendon is contained within a double-walled cylinder whose cavity is filled with a 

 fluid serving to diminish friction during the movements of the tendon (Fig. 492). It 

 is customary to distinguish the synovial portion of a tendon-sheath as the serous or 

 synovial sheath ( vagbia m ucosa) ivom the fibrous sheath 

 (vag-ina fibrosa) with which it is always closely con- Fig. 



■f 1 Fibrous 



nected. _ _ _ sheath 



Strands of connective tissue pass at intervals across ~ , If t^-" -^ ^^ Synovial 

 the synovial cavity of the sheath from the floor of the TV^^^^JtL ,.?'^^^''^ 



•^ 111 -111 1 1 ' ^>w'%-.i^T~ V inculutn 



groove on the bone and transmit blood-vessels to the 



tendon ; these strands constitute what are termed vin- Phalanx 



cula tendinum, or, from their general similarity to the 



mesentery, mesotcndonS. Diagram showing relations of ten- 



T i- _ J t- iU i don to tendon-sheath as in cross-section 



In some cases a tendon-sheath may serve to a cer- of finger. 

 tain extent as a pulley, affording a smooth surface over 



which the tendon changes its direction, as in the case of the extensor tendons of the 

 hand when this is partly extended. A special development of this condition is to be 

 seen in the tendinous loop {trochlea muscjclaris) over which the tendon of the superior 

 oblique muscle of the eyeball is reflected (Fig. 516). 



Bursae. — The intervals between the various muscles and between these or their 

 tendons and the bone are occupied by loose areolar tissue. In situations in which 

 a muscle or tendon in its movements comes in contact with a bony prominence, or in 

 which two tendons glide upon each other, the spaces of the areolar tissue enlarge and 

 become filled by a fluid resembling that of the synovial cavities, the result being the 

 formation of what is termed a bursa, whose purpose is to diminish the friction between 

 the muscle or tendon and the bone. Examples of such bursae are to be found abun- 

 dantly in connection with the muscles of the limbs, and some of those which occur 

 in the vicinity of joints frequently fuse with the adjacent synovial cavities ; the bursa 

 of the subscapularis, situated between that muscle and the neck of the scapula, for 

 instance, uniting with the synovial cavity of the shoulder-joint, and the bursa supra- 

 patellaris, between the tendon of the quadriceps femoris and the femur, fusing with 

 the cavity of the knee-joint. 



Bursae are also developed in the areolar tissue intervening between the superficial 

 and deep fasciae in situations in which the integument rests directly upon a bone, as, 

 for instance, over the olecranon process, and is frequently subjected to pressure in 

 that region. Such bursae are termed subcutaneous burses to distinguish them from 

 those developed in connection with the muscles. 



Classification of the Muscles. — The muscles may be classified according 

 to three plans : they may be arranged according to their topographical relations, 

 according to their physiological significance, or, finally, upon a morphological basis, 

 their embryologisal or developmental significance forming the guide for their arrange- 

 ment in groups. In the following pages the last-named plan will be followed as far 

 as possible. 



Embryologically the skeletal muscles are formed, for the most part, from a series 

 of segmentally arranged masses of mesoblast — the inesoblastic somites — which appear 

 at an early stage of development on either side of the notochord and later extend 

 ventrally towards the mid- ventral line (page 465). That portion of the musculature 

 which has such an origin may be regarded as consisting primarily of a series of plates 

 arranged segmentally along each side of the body, each plate corresponding to and 

 being supplied by one of the segmental nerves and by those fibres of it which arise 

 from the cells of the anterior horn of the spinal cord or their homologues in other 

 portions of the central nervous system. A diagrammatic representation of this mus- 

 culature in its primary condition is shown in Fig. 493, and from this it will be per- 

 ceived that the series of muscle-plates extends throughout the entire trunk and neck 



