338 ZOOTOMY. 



crico-arytenoids, and consisting of fibres passing transversely between the 

 arytenoid cartilages, which are approximated by their contraction. 



386. The anterior crico-arytenoid muscles, arising from the 

 lateral regions of the cricoid and inserted into the arytenoicls : to see 

 them one of the alae of the thyroid should be disarticulated and 

 reflected. 



387. The thyro-arytenoid muscles, also seen by reflection of the 

 thyroid : they run parallel and external to the vocal cords, arising 

 from the arytenoids and being inserted into the thyroid, which they 

 elevate. 



XLIX. Dissect away enough of the muscles in the 

 regions of the shoulder and hip to make out : 



388. The brachial plexus, formed by the union of the 

 fifth to eighth cervical and of the first thoracic nerves, 1 and 

 giving off nerves to the arm and shoulder. 



Besides several smaller nerves there are four chief trunics given 

 off from the brachial plexus : 2 (a) the ulnar nerve runs alongside the 

 brachial artery, passes im nediately internal to the olecranon into the 

 fore-arm, and then aljng the outer or ulnar side of the latter : (b) the 

 median nerve passes internal to the humerus, entering the fore-arm 

 proximal to the condyles, and takes a course along the inner or radial 

 side of the fore-arm ; both it and the ulnar nerve supply mainly the 

 flexor muscles : (c) the musculo-spiral nerve, the largest of the four, 

 goes to the dorsal side of the humerus and along the radial side of the 

 fore-arm ; it supplies mainly the extensor muscles : (cf) the circumflex 

 or subscapular nerve passes dorsalwards round the head of the 

 humerus and supplies some of the muscles of the shoulder. 



389. The lumbo-sacral plexus, formed by the 

 union of the fifth to seventh lumbar and of the first to 



1 There is a certain ambiguity in the usual mode of counting the 

 spinal nerves : in the cervical region each nerve is named from the 

 vertebra in front of which it emerges, the first or sub-occipital nerve 

 coming out between the skull and the atlas, the eighth between the 

 seventh cervical and first thoracic vertebra; : the remaining nerves are 

 named from the vertebras behind which they emerge : the first thoracic, 

 for instance, is the nerve which makes its exit between the first and 

 second thoracic vertebrae. 



; If the muscles of the arm are to be dissected in the same specimen, 

 the tracing of these nerves must be deferred. 



