288 A MANUAL OF ANATOMY 



clavian. It passes backwards beneath the levator anguH scapulae, 

 and then downwards beneath the rhomboid muscles, lying close to 

 the base of the scapula. It gives off branches to the adjacent 

 muscles, and several offsets to the front and back of the scapula, 

 which latter take part in the scapular anastomoses of arteries. 



At the upper border of the scapula a limited view is obtained of 

 the posterior belly of the omo-hyoid muscle, and the suprascapular 

 artery and nerve. The former arises from the upper border of the 

 bone inside the suprascapular notch, as well as from the supra- 

 scapular or transverse ligament. The suprascapular artery passes 

 to the supraspinous fossa over the ligament, and the suprascapular 

 nerve to the same region beneath it. The artery in this part of its 

 course furnishes its supra- acromial branch to the upper surface of the 

 acromion process. The transverse cervical artery is seen, at a higher 

 level than the suprascapular, dividing into its superficial cervical and 

 posterior scapular branches. The former passes superficial to the jf 

 levator anguli scapulae, and the latter beneath that muscle. 



It will now be convenient to continue the description of the 

 back to its termination, except the structures more immediately 

 involved in the head and neck. 



Serratus Posticus Superior — Origin. — The lower part of the liga- 

 mentum nuchae, and the spines and supraspinous ligaments of the 

 last cervical and first two thoracic vertebrae. 



Insertion. — By fleshy and tendinous slips into the upper borders 

 and outer surfaces of the second, third, fourth, and fifth ribs, external 

 to their angles. 



Nerve-supply. — The second, third, and fourth intercostal nerves. 



The origin of the muscle is aponeurotic, and continues so over 

 about half its length. The direction of the fibres is downwards 

 and outwards. 



Action. — ^The muscle elevates the ribs into which it is inserted, 

 and is therefore a muscle of inspiration. 



Serratus Posticus Inferior — Origin. — The posterior lamina of the 

 lumbar aponeurosis, through means of which the muscle is attached 

 to the spines and supraspinous ligaments of the lower two thoracic 

 and upper two or three lumbar vertebrae, under cover of the 

 latissimus dorsi. 



Insertion. — By four fleshy slips into the lower borders of the last 

 four ribs. 



Nerve-supply. — The ninth, tenth, and eleventh intercostal nerves. 



The serrations of insertion overlap each other from above down- 

 wards ; the second is the broadest, and the third to a large extent 

 conceals the fourth. The direction of the fibres is upwards and 

 outwards. 



Action. — (i) To draw backwards and slightly depress the lower 

 four ribs, the effect of which is to increase the capacity of the 

 lower part of the thoracic cavity ; and (2) to steady the lower four 

 ribs, and thus to act as an auxiliary to the diaphragm. In both 

 cases it acts as a muscle of inspiration. 



