CHAP. IV] THE GENERAL ANATOMY OF THE CERCOPITHECIDAE 133 



2. The M. digastricus is remarkable chiefly for the tendinous 

 arc stretched across the middle line of the neck (from one tendon 

 to the other). From this arcade the muscle-fibres of the anterior 

 belly take origin, the muscles of the right and left sides forming a 

 continuous sheet. The latter is however distinctly thick where it 

 is attached to the mandible. Here it lies immediately beneath the 

 lower border of the bone, but no "digastric" impression can be dis- 

 tinguished in the dry bone. The posterior belly is accompanied 

 by the M. stylo-hyoideus which divides to allow the tendon of the 

 M. digastricus to pass forward. 



3. The M. sterno-mastoideus is massive, and it arises from 

 the clavicle as well as the presternum. Thus two muscles are to 

 be distinguished, and they are nearly equal in size. 



4. The M. trapezius is attached to the outer fifth of the 

 clavicle, and is therefore widely separated here from the M. stemo- 

 mastoideus. Beneath the acromial part of the muscle, a second 

 and thicker stratum passes upwards to the cervical vertebrae 

 (M. acromio-trachelianus). Similarly, a M. occipito-scapularis 

 blends with the rhomboid sheet, which is thus carried up to the 

 skull. Lastly the M. levator anguli scapulae may be found in 

 continuity with the M. serratus magnus. Such muscles provide 

 useful stays for the fixation of the shoulder in climbing movements. 



The M. deltoideus occupies the whole of the ventral surface of 

 the clavicle (no clavicular component of the M. pectoralis major 

 being developed). This muscle will be found to extend also along 

 the whole length of the scapular spine as far as the vertebral 

 border of the bone. 



The M. pectoralis major is thus " sternal " only in origin : but 

 its presternal component is now of considerable size. The M. pec- 

 toralis minor arises likewise from the sternum (not from the ribs) 

 and it is inserted into a fascial sheet laid on the coracoid process, 

 but not strongly attached thereto. A third pectoral muscle (pos- 

 sibly fused in Man with the great pectoral muscle) arises from the 

 lower costal cartilages, and passes alongside the lesser pectoral 

 mass into a fascial expansion over the shoulder-joint. The sub- 

 clavius muscle is distinct or large. The M. latissimus dorsi is 

 characterized by its accessory, viz. the M. dorsi-epitrochlearis or 

 M. latissimo-condyloideus, seen in the Lemurs and Tarsius. The 



