OF THE BACK 409 



Nerves. The Rhomboid muscles are supplied by branches from the anterior division of 

 the fifth cervical nerve; the Levator anguli scapuly, by the anterior divisions of the third and 

 fourth cervical nerves, and frequently by a branch from the nerve to the Rhomboids. 



Actions. The movements effected by the preceding muscles are numerous, as may be con- 

 ceived from their extensive attachment. The whole of the Trapezius when in action retracts 

 the scapula and braces back the shoulder; if the head is fixed, the upper part of the Trapezius 

 will elevate the point of the shoulder, as in supporting weights; when the lower fibres are brought 

 into action, they assist in depressing the bone. The middle and lower fibres of the muscle rotate 

 the scapula, causing elevation of the acromion process. If the shoulders are fixed, both Trapezii, 

 acting together, will draw the head directly backward; or if only one acts the head is drawn to 

 the corresponding side. The Latissimus dorsi, when it acts upon the humerus, depresses it, 

 draws it backward, adducts, and at the same time rotates it inward. It is the muscle which 

 is principally employed in giving a downward blow, as in felling a tree or in sabre practice. 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 suspending and drawing the whole trunk for- 

 ward, as in climbing or walking on crutches. The Levator anguli scapulae raises the superior 

 angle of the scapula, and by so doing depresses the point of the shoulder. It assists the Trape- 

 zius in bearing weights and in shrugging the shoulders. If the shoulder be fixed, the Levator 

 anguli scapulae inclines the neck to the corresponding side and rotates it in the same direction. 

 The Rhomboid muscles carry the inferior angle backward and upward, thus producing a slight 

 rotation of the scapula upon the side of the thorax, the Rhomboideus major acting especially 

 on the lower angle of the scapula through the tendinous arch by which it is inserted. The Rhom- 

 boid muscles, acting together with the middle and inferior fibres of the Trapezius, will draw 

 the scapula directly backward toward the vertebral column. 



The Third Layer. 



Serratus posticus superior. Serratus posticus inferior. 



c i f Splenius capitis. 

 bplemus \ ~f\ 



( Splenius 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 Rhom- 

 boid muscles near their origin, and reflecting them downward, and by 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 (m, serratus posterior superior) is a thin, flat 

 quadrilateral muscle situated at the upper and back part of the thorax. It arises 

 by a thin and broad aponeurosis from the ligamentum nuchae, and from the 

 spinous processes of the last cervical and two or three upper thoracic vertebra? and 

 from the supraspinous ligament. Inclining downward and outward, 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. 



The Serratus posticus inferior (m. serratus posterior inferior) (Fig. 312) is 

 situated at the junction of the thoracic and lumbar regions; it is of an irregularly 

 quadrilateral form, broader than the preceding, and separated from it by a con- 

 siderable interval. It arises by a thin aponeurosis from the spinous processes of 

 the last two thoracic and two or three upper lumbar vertebrae, and from the supra- 

 spinous ligaments. Passing obliquely upward and outward, it becomes fleshy, 

 and divides into four flat digitations, which are inserted into the lower borders of 

 the four lower ribs, a little beyond their angles. The thin aponeurosis of origin 

 is intimately blended with the lumbar fascia. 



The vertebral aponeurosis is a thin, fibrous lamina, extending along the whole 

 length of the back part of the thoracic region, serving to bind down the long 

 Extensor muscles of the back which support the vertebral column and head, and 

 separate them from those muscles which connect the vertebral column to the upper 



