DORSAL AND LUMBAR FASCIA. SPLENIUS. 313 



The serratus posticus inferior (fig. 227, p. 207), broader and stronger than 

 the superior, arises from the spines of the lower two dorsal and upper two or three 

 lumbar vertebrse, by a thin aponeurotic membrane which forms part of the posterior 

 layer of the lumbar aponeurosis, and is closely united to the overlying tendon of the 

 latissimus dorsi. Passing outwards, upwards and forwards, it is inserted by four 

 fleshy digitations into the lower borders of the last four ribs. The fleshy digitations 

 are shorter than the aponeurotic part of the muscle, and they overlap one another 

 from above downwards. The last slip varies in size with the length of the twelfth 

 rib, and is often entirely concealed by the one above it. 



Varieties. Absence of the first or last digitation is. frequently seen ; of the whole 

 muscle very rarely. 



Nerves. The serrati muscles are supplied by the intercostal nerves, each slip receiving 

 n small branch which perforates the external intercostal muscle upon which it lies. 



Actions. The serratus posticus superior elevates the upper ribs, and is therefore a muscle 

 of inspiration. The serratus inferior draws the lower ribs downwards, and, to a greater 

 extent, backwards (see p. 162), and is also a muscle of inspiration, enlarging the lower part 

 of the chest, and at the same time resisting the tendency of the diaphragm to draw the 

 lower ribs upwards and forwards. 



DORSAL AND LUMBAR FASCIJE. The vertebral aponeurosis is situated on 

 the same plane as the serratus posticus inferior, and consists of a thin lamella which 

 separates the muscles belonging to the shoulder and arm from those which support 

 the spine and head. Its fibres are for the most part transverse, a few only being 

 longitudinal. Above, it passes beneath the superior serratus ; below, it is blended 

 with the tendons of the latissimus and inferior serratus muscles ; and in being 

 stretched from the spinous processes outwards to the angles of the ribs, it helps to 

 enclose the angular space in which are lodged the long extensor muscles. 



The transverse fibres of the vertebral aponeurosis represent the middle portion of the 

 muscular sheet of the serrati, the whole forming a continuous muscle in some of the lower 

 animals (Gegenbaur). 



Under the name of lumbar fascia or aponeurosis it has been customary to 

 describe three membranous layers ensheathing the erector spinae and quadratus 

 lumborum muscles. The deeper parts of this structure, to which by some the name 

 of lumbar fascia is restricted, are described along with the abdominal muscles, with 

 which they are chiefly connected (pp. 333 and 337). The superficial or posterior 

 layer (fig. 293, 10) is that through which the latissimus dorsi and serratus posticus 

 inferior muscles are attached to the spines of the vertebrse. This layer is of consider- 

 able strength, and extending outwards beyond the line along which the latissimus 

 becomes free from it, inclines forwards to join the middle layer, thus binding down 

 firmly the erector spinse muscle : it is by some described as the lower part of the 

 vertebral aponeurosis, with which it is continuous, by others it has been named the 

 aponeurosis of the latissimus dorsi. 



The SPLENIUS muscle is so named from its having the form of a strap, which 

 binds down the parts lying under it. It is attached superiorly in part to the 

 cervical vertebrse, in part to the skull, and is described accordingly under two names. 



a. The splenius colli is attached inferiorly to the spinous processes of the third, 

 fourth, fifth and sixth dorsal vertebrae, and superiorly along with the slips of the levator 

 auguli scapulae to the transverse processes of the first two or three cervical vertebrae. 



b. The splenius capitis, broader and thicker than the preceding, arises from 

 the spines of the seventh cervical and upper two dorsal vertebrae, and from the 

 ligamentum nuchae as high as the third cervical vertebra. It is inserted into the 

 lower part of the mastoid process, and into the outer part of the superior curved 

 line of the occipital bone. 



