434 THE MUSCLES AND FASCIAE 



Nerves. The Splenius is supplied from the external branches of the posterior 

 divisions of the cervical nerves ; the Serratus posticus superior is supplied by the 

 external branches of the posterior divisions of the upper dorsal nerves ; the Serratus 

 posticus inferior by the external branches of the posterior divisions of the lower 

 dorsal nerves. 



Actions. The Serrati are respiratory muscles. The Serratus posticus supe- 

 rior elevates the ribs ; it is therefore an inspiratory muscle ; while the Serratus 

 inferior draws the lower ribs downward and backward, and thus elongates the 

 thorax. It also fixes the lower ribs, thus aiding the downward action of the 

 diaphragm and resisting the tendency which it has to draw the lower ribs upward 

 and forward. It must therefore be regarded as a muscle of inspiration. This muscle 

 is also probably a tensor of the vertebral aponeurosis. The Splenii muscles of the 

 two sides, acting together, draw the head directly backward, assisting the Trapezius 

 and Complexus ; acting separately, they draw the head to one or the other side, and 

 slightly rotate it, turning the face to the same side. They also assist in supporting 

 the head in the erect position. 



Fourth Layer. 



Sacral and Lumbar Regions. Cervical Region. 



Erector spinse. Cervicalis ascendens. 



Dorsal Region. Transversalis colli. 



Ilio-costalis. Trachelo-mastoid. 



Musculus accessorius ad ilio-costalem. Complexus. 



Longissimus dorsi. Biventer cervicis. 



Spinalis dorsi. Spinalis colli. 



Dissection. To expose the muscles of the fourth layer, remove entirely the Serrati and the 

 vertebral and lumbar fasciae. Then detach the Splenius by separating its attachment to the 

 spinous processes and reflecting it outward. 



The Erector spinae (Fig. 288) and its prolongations in the dorsal and cervical 

 regions fill up the vertebral groove on each side of the spine. It is covered in 

 the lumbar region by the lumbar fascia ; in the dorsal region, by the Serrati 

 muscles and the vertebral aponeurosis ; and in the cervical region, by a layer of 

 cervical fascia continued beneath the Trapezius and the Splenius. This large 

 muscular and tendinous mass varies in size and structure at different parts of the 

 spine. In the sacral region the Erector spinse is narrow and pointed, and its origin 

 chiefly tendinous in structure. In the lumbar region the muscle becomes enlarged, 

 and forms a large fleshy mass. In the dorsal region it subdivides into two parts, 

 which gradually diminish in size as they ascend to be inserted into the vertebrae 

 and ribs. In the cervical region it is gradually lost, where a number of small muscles 

 are continued upward to the head to support it upon the spine. 



The Erector spinae arises from the sacro-iliac groove, and from the anterior 

 surface of a very broad and thick tendon, which is attached, internally, to the 

 spines of the sacrum, to the spinous processes of the lumbar vertebrae, and the 

 supraspinous ligament ; externally, to the back part of the inner lip of the crest of 

 the ilium, and to the series of eminences on the posterior part of the sacrum, 

 which represents the transverse processes, where it blends with the great sacro- 

 sciatic ligament. The muscular fibres form a single large fleshy mass, bounded in 

 front by the transverse processes of the lumbar vertebrae and by the middle lamella 

 of the lumbar fascia. Opposite the last rib it divides into two parts, the Ilio- 

 costalis and the Longissimus dorsi. 



The Ilio-costalis (Sacro-lumbalis), the external and smaller portion of the 

 Erector spinae, is inserted, generally, by six or seven flattened tendons into the 

 angles of the six or seven lower ribs. The number of the tendons of this muscle 

 is, however, very variable, and therefore the number of ribs into which it is 

 inserted. Frequently it is found to possess nine or ten tendons, and sometimes as 

 many tendons as there are ribs, and is then inserted into the angles of all the ribs. 



