202 MUSCLES OF THE BACK. 



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

 ribs. 



Relations. By its superficial surface with the trapezius, rhomboideus 

 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 processes and inter- 

 spinous ligaments of the two last dorsal and three 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 tendi- 

 nous 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 spinse, the intercostal 

 muscles and lower ribs. The upper border is continuous with a thin ten- 

 dinous 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 at- 

 tached mesially to the spinous processes of the dorsal vertebra*, and exter- 

 nally to the angles of the ribs ; superiorly it is continued upwards beneath 

 the serratus posticus superior, with the lower border of which it is some- 

 times connected. It serves to bind down the erector spinaa, and separate 

 it from the superficial 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 MUSCLE is single at its origin, but divides soon after into 

 two portions, w r hich are destined to distinct insertions. It arises by the 

 lower half of the ligamentum nuchaB, from the spinous processes of the 

 five last cervical, and from the spinous processes and interspinous liga- 

 ments of the six upper dorsal vertebrae ; it divides as it ascends the neck 

 into the splenius capitis and colli. The splenius capitis is inserted into 

 the rough surface of the occipital bone between the two curved lines, and 

 into the mastoid portion of the temporal bone. 



The splenius colli is inserted into the posterior tubercles of the trans- 

 verse processes of the three or four upper cervical vertebrae. 



Relations. By its superficial surface with the trapezius, sterno-mastoid, 

 levator anguli scapula?, rhomboideus minor and major, and serratus pos- 

 ticus superior. By its deep surface with the spinalis dorsi, longissimus dorsi, 

 semi-spinalis colli, complexus, trachelo-mastoid, and transversalis colli. 

 The tendons of insertion of the splenius colli are interposed between the 

 insertions of the levator anguli scapulae in front, and the transversalis colli 

 behind. 



The splenii of opposite sides of the neck leave between them a trian- 

 gular interval, in which the complexus is seen. 



Fourth Layer. 



Dissection. The two serrati and two splenii muscles must be removed 

 by cutting them away from their origins and insertions, to bring the fourth 

 layer into view. 



Three of these muscles, viz. sacro-lumbalis, longissimus dorsi, and 



