SCALENE MUSCLES 233 



It forms part of the floor of the posterior triangle of the 

 neck. Its superficial surface is in relation with the brachial 

 plexus and the third part of the subclavian artery. Its 

 posterior border touches the levator scapulae ; and the dorsal 

 scapular nerve and the descending branch of the transverse 

 cervical artery pass between it and that muscle. The lower 

 part of its anterior border is in relation with the apex of 

 the pleura, and the upper two roots of the long thoracic 

 nerve pierce the substance of the muscle. 



Musculus Scalenus Posterior. The scalenus posterior is 

 generally inseparable, at its origin, from the scalenus medius. 

 It is the smallest of the three, and springs by two or three 

 slips from the transverse processes of a corresponding number 

 of the lower cervical vertebrae, in common with the scalenus 

 medius. It is inserted into the upper border of the second 

 rib, immediately anterior to the insertion of the levator 

 costae and behind the large rough area which marks the 

 origin of the serratus anterior (Fig. 84). 



The scalene muscles are supplied by twigs from the 

 anterior branches of the cervical nerves^ particularly the lower 

 four. They elevated the ribs to which they are attached 

 and are, therefore, muscles of thoracic respiration. 



Dissection. The little muscle termed the rectus capitis 

 lateralis should now be cleaned, and its attachments defined. 

 It lies in the interval between the transverse process of the atlas 

 and the jugular process of the occipital bone, posterior to the 

 commencement of the internal jugular vein. The anterior 

 ramus of the first cervical nerve will be seen emerging from 

 under cover of its medial margin. 



Rectus Capitis Lateralis. The rectus lateralis arises from 

 the anterior part of the upper surface of the extremity of the 

 transverse process of the atlas, and is inserted into the under 

 surface of the jugular process of the occipital bone. It is 

 supplied by a twig from the anterior ramus of the first cervical 

 nerve. 



Dissection. By the time that the dissectors of the head and 

 neck have arrived at this stage of their work, the dissectors of 

 the thorax have, in all probability, finished their dissection. 

 If that is the case, the head and neck may be removed from the 

 trunk by cutting through the vertebral column at the level of 

 the intervertebral fibre-cartilage between the third and fourth 

 thoracic vertebrae. By this proceeding the upper three thoracic 

 vertebrae, with the attached portions of the first, second, and 

 third pairs of ribs, are removed with the neck. The scalene 

 muscles and the longus colli are therefore preserved intact. 



