THE VAGO-ACCESSORY MUSCLES. 



501 



a well-marked oval or rhomboidal tendinous area, which has been termed the oval 

 aponeurosis. 



In their course to their insertion the lower fibres pass over the smooth surface 

 at the base of the spine of the scapula, and sometimes a bursa mucosa is developed 

 between the bone and the muscle. 



Nerve-Supply. — From the external branch of the spinal accessory and from 

 the third and fourth cervical nerves. 



Action. — Acting from above, the upper fibres draw upward the point of the 

 shoulder, while, acting from below, they draw the head backward. The middle and 



lower fibres draw the scapula towards the 

 mid-dorsal line and at the same time 

 rotate it so as to raise the point of the 

 shoulder. 



Variations. — Like the sterno-mastoid, the 

 trapezius is a compound muscle consisting of 

 three distinct portions. That portion of the 

 muscle which inserts into the tuberosity of the 



Fig. 512. 



Sterno-mastoid 



Aponeurosis of trapezius 

 :• Trapezius 



-Acromion 



Si apular 

 spine 



Infraspinatus 



Rhoniboideus major 



Teres major 



Fig. 513. 



Latissimus dorsi 



Superficial dissection of back, showing trapezius 

 and adjacent muscles. 



do-occipitalis 



•Cleido-occipitalis cervicalis 

 Clavicle 



Acromion 



Dorso-scapularis 

 superior 



Tuberosity ot" spine 



Tendinous slip to 



infraspinous fascia 

 Dorso-scapularis inferior 



Latissimus dorsi 



Components of human trapezius muscle. 



{Styeissley.) 



scapular spine represents what is termed in the lower mammals the dorso-scapularis inferior, 

 while the portiort which inserts into the spine and acromion process represents the dorso-scap- 

 ularis superior. The clavicular portion, on the other hand, is in the lower forms associated with 

 the cleido-occipitalis element of the sterno-cleido-mastoid, and may therefore be termed the 

 cleido-occipitalis cervicalis. 



hidications of this triple constitution are to be seen in a more or less distinct separation of 

 the clavicular portion of the muscle from the rest and, less frequently, by a separation of the 

 lower from the middle portion (Fig. 513). Occasionally, too, bundles pass from the anterior 

 border of the clavicular portion to join the cleido-occipitalis portion of the sterno-cleido-mastoid, 

 indicating the common origin of the two muscles. Variations likewise occur in the extent of 

 the spinal attachment of the trapezius, owing to the reduction of one or other of its parts, and it 

 may be remarked that this attachment usually extends lower in the muscle of the right side 

 than in that of the left. .. . ; 



Of especial interest from the comparative stand-point is the occasional existence of a bundle 

 of fibres which lies beneath the cervical portion of the trapezius, and is attached at one extremity 

 to the outer end of the clavicle or to the acromion process and above to the transverse processes 

 of some of the upper cervical vertebrae, usually the atlas and axis. It is apparently the equivalent 

 of the omo-transversarius of the lower mammals, a muscle which is closely associated with the 

 members of the trapezius group. 



