200 MUSCLES OF THE BACK. 



Third Layer. 



Dissection. The third layer consists of muscles which arise from 

 the spinous processes of the vertebral column, and pass outwards. It 

 is brought into view by dividing the levator anguli scapulae near its 

 insertion, and reflecting the two rhomboid muscles upwards from their 

 insertion into the scapula. The latter muscles should now be removed. 



The SERRATUS POSTICUS SUPERIOR is situated at the upper part of 

 the thorax ; it arises from the ligamentum nuchag, the spinous process 

 of the last cervical and those of the two upper dorsal vertebrae. The 

 muscle passes obliquely downwards and outwards, and is inserted by 

 four serrations into the upper border of the second, third, fourth, and 

 fifth ribs. 



Relations. By its superficial surface with the trapezius, rhom- 

 boideus major and minor, and serratus magnus. By its deep surface 

 with the splenius, the upper part of the erector spinae, the intercostal 

 muscles and ribs. 



The SERRATUS POSTICUS INFERIOR arises from the spinous pro- 

 cesses and interspinous ligaments of the two last dorsal and two upper 

 lumbar vertebrae, and passing obliquely upwards is inserted by four 

 serrations into the lower border of the four lower ribs. Both muscles 

 are constituted by a thin aponeurosis for about half their extent. 



Relations. By its superficial surface with the latissimus dorsi, its 

 tendinous origin being inseparably connected with the aponeurosis of 

 that muscle. By its deep surface with the aponeurosis of the obliquus 

 internus, with which it is also closely adherent; with the erector 

 spinae, the intercostal muscles and lower ribs. The upper border is 

 continuous with a thin tendinous layer, the vertebral aponeurosis. 

 The Vertebral aponeurosis is a thin membranous expansion composed 

 of longitudinal and transverse fibres, and extending the whole length 

 of the thoracic region. It is attached mesially to the spinous pro- 

 cesses of the dorsal vertebrae, and externally to the angles of the ribs ; 

 superiorly it is continued upwards beneath the serratus posticus supe- 

 rior, with the lower border of which it is sometimes connected. It 

 serves to bind down the erector spinae, and separate it from the super- 

 ficial muscles. 



The serratus posticus superior must be removed from its origin and 

 turned outwards, to bring into view the whole extent of the splenius 

 muscle. 



The SPLENIUS M USCLE is single at its origin, but divides soon after 

 into two portions, which are destined to distinct insertions. It arises 

 from the lower half of the ligamentum nuchae, from the spinous pro- 

 cess of the last cervical, and from the spinous processes and interspinous 

 ligaments of the six upper dorsal vertebrae ; it divides as it ascends 

 the neck into the splenius capitis and colli. The splenius capitis is in- 

 serted into the rough surface of the occipital bone between the two 

 curved lines, and into the mastoid portion of the temporal bone. 



