296 A MANUAL OF ANATOMY 



intertransversales mediates extend from the accessory process of 

 one vertebra to the mammillary process of the vertebra below. 

 Sometimes intertransversales muscles are met with in the lower 

 thoracic region. 



Nerve-supply. — Posterior primary divisions of spinal nerves. 



Action. — ^The muscles act as lateral flexors of the vertebral 

 column. 



Levatores Costarum. — ^These are twelve in number on either 

 side. 



Origin. — The highest muscle arises from the tip of the transverse 

 process of the seventh cervical vertebra, and the succeeding eleven 

 arise from the tips of the transverse processes of the thoracic verte- 

 brae from the first to the eleventh inclusive. 



Insertion. — Each muscle is inserted into the outer surface of the 

 rib below, from the tubercle to the angle. In the case of the lower 

 two or three muscles the more superficial fibres pass over the first 

 rib below and take insertion into the next rib, these fibres con- 

 stituting the levatores costarum longiores. 



Nerve-supply. — ^The intercostal nerves. 



The muscles are directed downwards and outwards. 



Action. — ^To elevate the ribs, as in inspiration. 



Each muscle is somewhat fan-shaped, and contains an admixture 

 of aponeurotic fibres. In direction the muscles coincide with the 

 external intercostals, with which they are closely incorporated 

 by their outer borders. They are covered by the erector spinae, 

 and by their deep surfaces they are related to the external inter- 

 costals. 



Posterior Primary Divisions of Spinal Nerves. — These nerves in 

 the thoracic region pass backwards, each through a four-sided space 

 bounded below by the neck of a rib, above by the transverse process 

 of the upper vertebra, externally by the superior costo-transverse 

 ligament, and internally by the body of a vertebra. Between the 

 transverse processes each divides into an internal and external 

 branch. The internal branches incline inwards on the superficial 

 surface of the multifidus spinae, and the upper six become cutaneous 

 near the spines of the vertebrae, whilst the lower six end in the 

 deep muscles. The external branches pass outwards beneath the 

 middle column of the erector spinae, and, on reaching the interval 

 between the middle and outer columns of that muscle, they end 

 differently in the upper and lower parts of the back. The upper 

 six end in the deep muscles, but the lower six become cutaneous 

 along the line of the angles of the ribs. In the lumbar region the 

 posterior primary divisions pass backwards, each through a space 

 bounded externally by the quadratus lumborum muscle, internally 

 by the intortransversalis lateralis muscle, and above and below by 

 a transverse process. Their further disposition is as in the thoracic 

 region, but the external branches of the first three only furnish 

 cutaneous nerves, and, as already stated, these are principally 

 gluteal in their distribution. 



