274 MUSCLES AND FASCIJE. 



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

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

 Erector spinaa, Intercostal muscles and ribs. 



The Serratus Posticus Inferior is situated opposite the junction of the dorsal 

 and lumbar regions, is of an irregularly quadrilateral form, broader than the pre- 

 ceding, 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 lower ribs, a little beyond 

 their angles. 



Relations. By its superficial surface, it is covered by the Latissimus dorsi, with 

 the aponeurosis of which its own aponeurotic origin is inseparably blended. By 

 its deep surface, with the posterior aponeurosis of the Transversalis, the Erector 

 spinae, ribs and Intercostal muscles. Its upper margin is continuous with the 

 vertebral aponeurosis. 



The vertebral aponeurosis is a thin aponeurotic lamina, extending along the 

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

 Erector spinas, and separating it 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 ; externally, to the angles of the ribs ; and 

 below, to the upper border of the Inferior serratus and tendon of the Latissimus 

 dorsi ; above, it passes beneath the Splenius, and blends with the deep fascia of 

 the neck. 



The Serratus posticus superior should now be detached from its origin and turned outwards, 

 when the Splenius muscle will be brought into view. 



The Splenius is situated at the back of the neck and upper part of the dorsal 

 region. At its origin, it is a single muscle, narrow and pointed in form ; but it 

 soon becomes broader, and divides into two portions, which have separate inser- 

 tions. It arises, by tendinous fibres, from the lower half of the ligamentum 

 nucbse, from the spinous processes of the last cervical and of the six upper dorsal 

 vertebras, and from the supra-spinous ligament. From this origin, the fleshy 

 fibres proceed obliquely upwards and outwards, forming a broad flat muscle, which 

 divides as it ascends into two portions, the Splenius capitis and Splenius colli. 



The splenius capitis is inserted into the mastoid process of the temporal bone, and 

 into the rough surface on the occipital bone beneath the superior curved line. 



The splenius colli is inserted, by tendinous fasciculi, into the posterior tubercles 

 of the transverse processes of the three or four upper cervical vertebras. 



The Splenius is separated from its fellow of the opposite side by a triangular 

 interval, in which is seen the Complexus. 



Relations. By its superficial surface, with the Trapezius, from which it is sepa- 

 rated below by the Ehomboidei and the Serratus posticus superior. It is covered 

 at its insertion by the Sterno-mastoid. By its deep surface, with the Spinalis 

 dorsi, Longissimus dorsi, Semi-spinalis colli, Complexus, Trachelo-mastoid, and 

 Transversalis colli. 



Nerves. The Splenius and Superior serratus are supplied from the external 

 posterior branches of the cervical nerves; the Inferior serratus, from the external 

 branches of the dorsal nerves. 



Actions. The Serrati are respiratory muscles acting in antagonism to each 

 other. The Serratus posticus superior elevates the ribs and is, therefore, an inspi- 

 ratory muscle; while the Serratus inferior draws the lower ribs downwards, and 

 is a muscle of expiration. This muscle is also probably a tensor of the vertebral 

 aponeurosis. The Splenii muscles of the two sides, acting together, draw the 

 head directly backwards, assisting the Trapezius and Complexus ; acting sepa- 

 rately, they draw the head to one or the other side, and slightly rotate it, turning 



