176 HEAD AND NECK 



costal arteries make their appearance between the transverse 

 processes. Each of these vessels reaches this point by 

 passing dorsally in the interval between the body of a vertebra 

 and the anterior costo-transverse ligament. It is associated 

 with the corresponding posterior branch of a spinal nerve, 

 and is distributed with it to the muscles and integument of 

 the back. 



In the lumbar region similar branches are derived from the 

 lumbar arteries. They are distributed in the same manner. 



In both dorsal and lumbar regions these vessels, before 

 reaching the back, furnish small spinal branches which enter 

 the vertebral canal through the intervertebral foramina. 

 These will be traced at a later period. 



The veins accompanying the dorsal branches of the lumbar 

 and intercostal arteries pour their blood into the lumbar 

 and intercostal veins. They are of large size, being joined 

 by tributaries from the posterior vertebral venous plexus, and 

 also by others from within the vertebral canal. 



Dissection. The remains of the third layer of spinal muscles must now 

 be dissected. They are the semispinalis dorsi and semispinalis cervicis. 



The semispinalis cervicis is already exposed ; but to display the 

 semispinalis dorsi it is necessary to remove the spinalis dorsi muscle. 



The semispinalis dorsi is composed of a series of muscular slips, with 

 long tendons at either end, which arise from the transverse processes of the 

 sixth to the tenth thoracic vertebra. It is inserted into the spines of the 

 upper four thoracic and lower two cervical vertebra. The semispinalis 

 cervicis lies under cover of the semispinalis capitis. It springs from the 

 transverse processes of the upper five thoracic vertebra, and is inserted into 

 the spines of the second to the fifth cervical vertebra. The slips composing 

 the semispinalis muscles stretch over five or more vertebra. 



Dissection. The fourth layer of muscles must now be examined. It 

 includes the multifidus, the rotatores, the interspinales, theintertransversales, 

 and the recti and oblique muscles of the sub-occipital region. The latter have 

 already been exposed by the reflection of the semispinalis capitis (complexus). 

 To display the other members of the group the semispinalis dorsi and 

 cervicis must be detached from the spines and drawn aside, and the 

 sacrospinalis must be separated from the lumbar and sacral spines and 

 turned laterally, if this has not already been done in tracing the nerves. 



Musculus Multifidus. In the lumbar and sacral regions the multi- 

 fidus will be seen to constitute a thick fleshy mass, which clings closely 

 to the vertebral spines. In this situation it has a very extensive origin 

 viz., (i) from the deep surface of the aponeurotic origin of the sacrospinalis ; 



(2) from the posterior surface of the sacrum as low as the fourth aperture ; 



(3) from the posterior sacro-iliac ligament ; (4) from the posterior superior 

 spine of the ilium ; and (5) from the mammillary processes of the lumbar 

 vertebrae. In the thoracic region it takes origin from the transverse processes 

 of the vertebrae, and in the cervical region from the articular processes of at 

 least four of the lower cervical vertebrae. The bundles which compose the 



