196 Mammalia, 



Aortic arch on the left side. The median nerve, so called from 

 the course it takes along the middle line of the arm to the 

 hand, arises by two roots from the Brachial Plexus, which the 

 student should now trace up, being very careful not to injure 

 any of the injected vessels^ We may here give a caution 

 which should in every case be observed, namely, always to 

 work with th6 scalpel along the line of direction of a vessel or nerve, 

 and never across it. The Brachial Plexus is formed by the 

 union of the anterior branches of the lower cervical and first 

 dorsal nerves, and extends from the lower part of the side of 

 the neck to the axilla, being plexiform only in one part, and 

 that not far from the neck. Around the plexus and the 

 axillary vessels may be found traces of lymphatic glands im- 

 bedded in the areolar tissue. 



Beneath the axillary vessels and nerves, with its deep 

 surface resting upon the ribs and intercostal spaces, is the 

 Serratus magnus muscle noted above ; this muscle is covered 

 in front by the Pectoralis major. The sternal digitations of 

 this muscle in part overlie the small serially repeated ex- 

 ternal Intercostal muscles which extend from the tubercles of 

 the ribs nearly to the outer end of the cartilages. The fibres 

 of the Inter costals are attached to the adjacent margins of each 

 pair of ribs, and are directed nearly parallel with the sternum. 

 The action of the external intercostals is to elevate the ribs . 

 they are, therefore, muscles of inspiration : in this action they 

 are aided by part of some deeper muscles just within the ribs> 

 termed the internal intercostals, viz., that part apparent 

 between the costal cartilages (from their outer end to the 

 sternum), and whose fibres have a general oblique direction 

 downwards (towards the sternum) and forwards (towards the 

 head), therefore, to some degree crossing those of the 

 external intercostals. The lateral portions of the internal 

 intercostals, underlying the external intercostals, act as de- 

 pressors of the ribs, and therefore aid in eorpiratory eflPorts. 



