296 



THE MUSCLES 



elevated and fixed, this muscle Avill draw the front of the chest up to it, as in 

 climbing. It will also raise the up})er ril)s in forced ins])iration. 



Relations. — Superficially, the ])latysma myoides, the mammary gland, and the 

 cutaneous nerves from the descending branches of the cervical plexus, as •well as 

 the lateral and anterior perforating branches of the upper intercostals. 



Beneath, lie the subclavius, ju^ctoralis minor, and serratus magnus, the fascia 

 covering the external and internal intercostal muscles, the coracoid process, and 

 costo-coracoid membrane, with the structures which pass through the membrane, 

 viz. the cephalic vein, acromio-thoracic vessels, and the external anterior thoracic 

 nerve. Further outwards it forms jjart of the anterior wall of the axilla, and enters 

 into relation with the biceps, the coraco-brachialis, and the axillar}' vessels and 

 nerves. 



The upper border is separated from the deltoid by the cephalic vein and the 

 humeral branch of the acromio-thoracic artery. 



Variations. — The clavicular and sterno-costal portions are often widely separated. They 

 may also be further subdivided into separate bauds or sheets, and they may be wholly or partly 

 absent. Occasionally, the muscles of the two sides join across the sternum, or muscular bauds 

 may unite the sternal ends of the clavicles. Bands may cross from the axillary border to the 

 latissimus dorsi or the deep fascia of the upper arm ; deeper bands may also pass to the insertion 

 of the pectoralis minor, or the capsule of the shoulder-joint. 



Second Layer 



1. SUBCLAVIUS 



The subclavius — named from its position beneath the clavicle (^ clavis) — is 

 almost cylindrical, but may be more accurately described as a thick sheet of the 

 shape of a low obtuse-angled triangle, the obtuse angle being contained between the 

 clavicular attachment and the inner free border of the muscle. 



Fig. 265. — The Subclavius and the Upper Portion of the Serratus Magnus. 



Subclavius 



Serratus maguua 



Origin. — The upper and anterior surface of the first rib and its cartilage at their 

 point of junction. 



Insertion. — The groove on the lower surface of the clavicle from the rhomboid 

 im])ression to the (;onoid tubercle. 



Structure. — It arises by a strong tendon, flattened from before backwards, 

 which lies close to the front of the rhomboid ligament, and is continued for a con- 

 siderable distance along the lower border of the muscle. Its insertion is by fleshy 

 fi))res which radiate upwards, outwards, and a little backwards, in a penniform 

 manner, from this tendon; the inner ascending more vertically, and the outer very 

 obli(|uely. 



Nerve-supply. — From the brachial plexus by a small lu-anch which, arising 

 from the fifth and sixth cervical nerves, passes behind the clavicle and enters the 

 middle of the back of the muscle. 



