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 arytenoids : to see 

 them one of the alse 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. 

 i 



XLTX. 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 trunKS given 

 oft" 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 along 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 vertebrse : the remaining nerves are* 

 named from the vertebrae behind which they emerge : the first thoracic, j 

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

 second thoracic vertebrae. 



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

 the tracing of these nerves must be deferred. 



