OF THE BACK. 341 



Trapezii, acting together, will draw the head directly backward ; or if only one 

 acts, the head is di*awn to the corresponding side. 



The Latissimus dorsi, when it acts upon the humerus, depresses it, draws it back- 

 ward, 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 forward, as in climbing or walking on 

 crutches. 



The Levator anguli scapulae raises the superior angle of the scapula, assisting 

 the Trapezius in bearing weights or 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 chest, the Rhomboideus major acting especially on the lower angle 

 of the scapula through the tendinous arch by which it is inserted. The Rhomboid 

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

 draw the scapula directly backward toward the spine. 



Third Layer. 



Serratus posticus superior. Serratus posticus inferior. 



01- f Splenius capitis. 

 bplenms< a^ . 



( 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 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 nuchge, and from the spinous processes of the last cervical 

 and two or three upper dorsal vertebrae 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. 



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

 Levator anguli scapulas ; by its deep surface, with the Splenius and the vertebral 

 aponeurosis, which separates it from the prolongations of the Erector spinae, 

 and with the 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 supraspinous 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. 



Relations. By its superficial surface, with the Latissimus dorsi. By its deep 

 surface, with the Erector spinae, ribs, and Intercostal muscles. Its upper margin 

 is continuous with the vertebral aponeurosis. 



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 spine and head, and separate them 

 from those muscles which connect the spine to the upper extremity. It consists of 

 longitudinal and transverse fibres blended together, forming a thin lamella, which 

 is attached in the median line to the spinous processes of the dorsal vertebrae ; ex- 



