OF THE BACK. 351 



Relations. By their superficial (posterior) surface, with the integument and 

 Trapezius ; the Ehomboideus Major, with the Latissimus Dorsi. By their deep 

 (anterior] surface, with the Serratus Posticus Superior, posterior scapular artery, 

 part of the Erector Spinae, the Intercostal muscles and ribs. 



Nerves. These muscles are supplied by branches from the fifth cervical 

 nerve, and additional filaments from the deep branches of the cervical plexus 

 are distributed to the Levator Anguli Scapulas. 



Actions. The movements effected by the preceding muscles are numerous, 

 as may be conceived from their extensive attachment. If the head is fixed, the 

 upper part of the Trapezius will elevate the point of the shoulder, as in sup- 

 porting weights ; when the middle .and lower fibres are brought into action, 

 partial rotation of the scapula upon the side of the chest is produced. If the 

 shoulders are fixed, both Trapezii acting together -will draw the head directly 

 backwards, or if only one acts, the head is drawn to the corresponding side. 



The Latissimus Dorsi, when it acts upon the humerus, draws it backwards 

 and downwards, and at the same time rotates it inwards. If the arm is fixed, 

 the muscle may act in various ways upon the trunk ; thus, it may raise the 

 lower ribs and assist in forcible inspiration, or if both arms are fixed, the two 

 muscles may assist the abdominal and great Pectoral muscles in drawing the 

 whole trunk forwards, as in climbing or walking on crutches. 



The Levator Anguli Scapulas raises the superior angle of the scapula after it 

 has been depressed by the lower fibres of the Trapezius, whilst the. Rhomboid 

 muscles carry the inferior angle backwards and upwards, thus producing a 

 slight rotation of the scapula upon the side of the chest. If the shoulder be 

 fixed, the Levator Anguli Scapulae may incline the neck to the corresponding 

 side. The Rhomboid muscles acting together with the middle and inferior 

 fibres of the Trapezius, will draw the scapula directly backwards towards the 

 spine. 



THIRD LAYER. 



Serratus Posticus Superior. Serratus Posticus Inferior. 



01- ( Splenius Capitis. 

 Splenms < 0*% n \i: 



( Splernus Coin. 



Dissection. To bring into view the third layer of muscles, remove the whole of the second, 

 together with the Latissimus Dorsi ; by cutting through the Levator Anguli Scapulae and 

 Rhomboid muscles near their insertion, and reflecting them upwards, to expose the Serratus 

 Posticus Superior, dividing the Latissimus Dorsi in the middle by a vertical incision carried 

 from its upper to its lower part, and reflecting the two halves of the muscle. 



The Serratus Posticus Superior is a thin, flat, quadrilateral muscle, situated 

 at the upper and back part of the thorax. It arises by a thin and broad apo- 

 neurosis, from the ligamentum nuchas, and from the spinous processes of the 

 last cervical and two or three upper dorsal vertebrae. Inclining downwards 

 and outwards, it becomes muscular, and is inserted, by four fleshy digitations, 

 into the upper borders of the second, third, fourth, and fifth ribs, a little beyond 

 their angles. 



Relations. By its superficial surface, with the Trapezius, Rhomboidei, and 

 Serratus Magnus. By its deep surface, with the Splenius, upper part of the 

 Erector Spinae, Intercostal muscles and ribs. 



The Serratus Posticus Inferior is situated at the junction of the dorsal and 

 lumbar regions: it is of an irregularly quadrilateral form, broader than the 

 preceding, and separated from it by a considerable interval. It arises by a 

 thin aponeurosis from the spinous processes of the last two dorsal and two or 

 three upper lumbar vertebrae, and from the interspinous ligaments. Passing 

 obliquely upwards and outwards, it becomes fleshy, and divides into four flat 

 digitations, which are inserted into the lower borders of the four iower ribs, a 

 little beyond their angles. 



