SERRATUS POSTICUS IXFERIOR— VERTEBRAL APONEUROSIS 413. 



Relations. — Superficially, the tra])ezius, levator aiiiruli scapulte, rhoiiiljoideus 

 minor and major; deeply, the vertel)ral aponeurosis which separates it from the 

 splenius and the more superficial spinal muscles; farther outwards the muscle 

 covers part of the external intercostals. 



Variations. — The .'^lips vary in number. Sometimes it gives oflF a slip from its upper border 

 or posterior surface which goes to the levator anguli scapulae, the mastoid process, or the superior 

 nuchal line. 



2. SERRATUS POSTICUS IXFERIOR 



The serratus posticus inferior — named also from its outer saw-like margin, 

 and its position — is a quadrilateral sheet. 



Origin. — The spines of the two lower thoracic and two upper lumbar vertebrae. 



Insertion. — The lower borders of the last four or five ribs external to their 

 anirlfs, except in the case of the last rib which has no angle; and as far forwards 

 as tliL' costal origins of the latissimus dorsi. 



Structure. — More than half of the muscle is aponeurosis, which is blended with 

 that of the latissimus dorsi on its superficial, and of the erector spinse on its deep 

 aspect, and forms a part of the vertebral aponeurosis, Avhich blends with the pos- 

 terior layer of the lumbar fascia. The fleshy fibres are directed upwards and out- 

 wards from this ajDoneurosis, arising in an oblic|ue line which is nearer the spine 

 above than below, and inserted directly, or with small tendinous attachments into 

 the ribs. Each succeeding tooth slightly overlaps, and extends farther forwards 

 than the one below. The middle teeth are the largest. 



Nerve-supply. — Filaments from the tenth and eleventh intercostals which 

 enter the deep surface of the outer part of the muscle. 



Action. — (1) To depress the lower ribs; (2) to fix them, so as to enable the 

 diaphragm to contract. It is therefore in this respect a muscle of inspiration. 



Relations. — Superficially, the latissimus dorsi; deeply, the erector spinse with 

 its upward continuations, also the levatores costarum, and the external intercostal 

 muscles. 



GENERAL ARRANGEMENT OF THE MUSCLES 

 ACTING UPON THE SPINAL COLU:\IN 



THE VERTEBRAL APONEUROSIS 



Beneath the serrati postici are arranged what may be called the muscles of the 

 back projjer, viz. those which move the vertebral column and act chiefly upon the 

 spines and transverse processes of the vertebrae and the parts of the skull which 

 are serially continuous with them. The}' lie for the most part in the groove on 

 either side of the spines of the vertebrae, and they are bovuid down and protected 

 by the strong vertebral aponeurosis which is blended below with the tendinous 

 origin of the latissimus dorsi, the serratus posticus inferior, and the erector spinae. 

 Laterally, it is attached to the angles of the ribs; and internally, to the spines of the 

 vertebrae and the supra-spinous ligament. Above, it passes beneath the tendon of 

 the serratus posticus superior, and is then lost in the thin fascia covering the splenii. 



The back muscles proper may l)e arranged according to their direction into 

 two great divisions, each of Avhich contains two classes. The vertical division 

 consists of muscles which run between similar points of bone, and its two classes 

 will contain the spinales, which connect the spines of the verteliraj with one another 

 and with the mesial portion of the occipital bone; the transrcrsales, which connect 

 the transverse processes of the sacral and lumbar regions with the transverse pro- 

 cesses and the adjacent portions of the ribs in the thoracic region, the articular and 

 transverse processes in the cervical region, and the mastoid process at the upi>er 

 end of the spinal column. The oblique division consists of muscles which run 

 obliquely, and contains two classes: the ■•<pino-tran.-<rersnIes, which run upwards and 

 outwards from spine to transverse process, or to a point of bone which corresponds 



