SCALENUS MEDI US— SCALENUS POSTICUS 455 



and laterally, and to rotate it so as to turn the face to tlie opposite side; (2) it will 

 raise the first rib, especially in forced inspiration. 



Relations. — In front lie the sterno-niastoid, omo-hyoid, and subclavius, the 

 internal jugulai vein, the subclavian vein and phrenic nerve; on the inner side are 

 the rectus cai)itis anticus major, longus colli, the vertebral vessels, and the sympa- 

 thetic cord; on the outer side and behind emerge the anterior primary branches of 

 the cervical nerves separating it from the scalenus medius; and loVer down it 

 crosses in front of the second part of the subclavian artery and the pleura. 



2. SCALENUS MEDIUS 



The scalenus medius — named from its shape and position — is an elongated 

 triangular sheet. 



Origin. — The upper surface of the first rib between the tuberosity and the groove 

 for the subclavian artery. 



Insertion. — The front of the posterior tubercles of the six lower cervical 

 vertebne, and frequently also the lower part of the lateral mass of the atlas. 



Structure. — Arising by a broad band, tendinous in front and muscular behind, 

 the fleshy fibres form a thick sheet of slightly divergent fibres, whicli run upwards 

 and inwards to end upon the vertebrae in six short tendons. 



Nerve-supply. — The anterior primary branches of the cervical nerves as 

 soon as they emerge supply numerous filaments to the inner part of its anterior 

 surface. 



Action. — Acting from below, to flex the neck laterally; acting from above, to 

 raise the first rib as in forced inspiration, or to fix it in ordinary inspiration. 



Relations. — In front lie the sterno-mastoid, omo-hyoid, and trapezius, the 

 subclavian artery, cervical and brachial plexuses; behind, it is in contact with the 

 levator anguli scapulae and scalenus posticus. 



3. SCALENUS POSTICUS 



The scalenus posticus — named from its form and position — is a triangular 

 sheet. 



Origin. — The upper part of the outer surface of the second rib l)ehind the 

 rough prominence for the serratus magnus. 



Insertion. — The lower surface of the posterior tubercles of the two or three 

 lowest cervical vertebrae. 



Structure. — Arising partly directly from the bone, and partly by a short apo- 

 neurosis which covers the outer and posterior part of the origin, its fleshy fibres 

 converge and are inserted by three short tendons. 



Nerve-supply. — Small filaments which come from the lower three cervical 

 nerves at their points of emergence, and, after passing through the scalenus medius, 

 enter the front service of the muscle. 



Action. — When the rib is fixed, to flex the lower part of the neck laterally, 

 and acting from aliove, to raise the second rib, especially in forced inspiration. 



Relations. — In front lies the scalenus medius; behind, the levator anguli 

 scapulae. 



Variations of the scaleni. — The scalenus posticus maybe absent. The scalenus medius may 

 arise as low as the third rib. A portion of the scalenus anticus may be separate from the rest, 

 and pass behind the subclavian artery. 



