62 ANATOMY FOli NUKSES. [CHAP. VI. 



The muscles of the back are disposed in five layers, one be- 

 neath another. The two largest and most superficial are the 

 trapezius and the latissimus dorsi. 



The trapezius arises from the middle of the occipital bone, 

 and from the spine as far as the last dorsal vertebra. From 

 this extended line of origin the fibres converge to their in- 

 sertion in the acromion process and spine of the scapula. 

 The latissimus dorsi arises from the last six dorsal vertebrae, 

 and through the medium of the lumbar aponeurosis, from the 

 lumbar and sacral part of the spine and from the crest of the 

 ilium. The fibres pass upwards and converge into a thick, nar- 

 row band, which winds around and finally terminates in a flat 

 tendon, which is inserted into the front of the humerus just 

 below its head. 



These muscles cover nearly the whole of the back ; but as 

 they act upon the bones of the upper extremity, they are often 

 more properly reckoned as belonging to the muscles of that 

 region. 



The muscles of the thorax are chiefly concerned with the 

 movements of the ribs during respiration. They. are the inter- 

 costals, subcostals, etc. 



The chief bulk of the anterior muscular wall of the chest is 

 made up of the pectoral muscles, which arise mainly from the 

 front of the sternum. The fibres converging form a thick mass, 

 which is inserted by a tendon of considerable breadth into the 

 upper part of the humerus. As these muscles move the arm, 

 they are, like the superficial muscles of the back, usually reck- 

 oned among ,the muscles of the upper extremity. Covering the 

 pectoral muscles is a superficial fascia (composed of connective 

 tissue) in which are lodged the mammary glands and a variable 

 amount of fat. 



The muscular walls of the abdomen are mainly formed by 

 three layers of muscles, the fibres of which run in different direc- 

 tions, those' of the superficial and middle layers being oblique, 

 and those of the innermost layer being transverse. In front, 

 these three layers of muscles are replaced by tendinous expan- 

 sions or aponeuroses, which meet in the middle line, the line of 

 union giving rise to a white cord-like line, the linea alba. On 

 each side of this line the fibres of a straight muscle, the rectus 

 muscle, extend in a vertical direction between the tendinous 



