I 3 i4 HUMAN ANATOMY. 



The external or radial branch inosculates with the musculo-cutaneous nerve and dis- 

 tributes filaments to the integument of the thenar eminence and the radial side of the thumb as 

 far out as the base of the nail. 



The internal or ulnar branch splits into two parts. The inner of these likewise under- 

 goes dichotomous division and supplies the dorsal aspect of the adjacent surfaces of the thumb 

 and the index finger. The outer divides similarly to the inner and is distributed to the adjoining 

 sides of the index and middle fingers. It gives off a branch which inosculates with the adjacent 

 filament from the dorsal branch of the ulnar nerve, so that the contiguous surfaces of the middle 

 and ring fingers are the recipients of fibres from both the radial and ulnar nerves. 



As the ulnar side of the hand is approximated the digital area of distribution of the radial 

 nerve gradually recedes toward the wrist. On the thumb the radial extends as far out as the 

 base of the nail, on the index finger as far as the middle of the second phalanx and on the 

 middle finger only over the proximal portion of the first phalanx. The deficiency in these 

 instances is supplied by twigs from the digital branches of the median nerve. 



Variations. The musculo-spiral may accompany the circumflex nerve through the quad- 

 rilateral space. It may communicate with the ulnar nerve in the upper arm. Cases are 

 recorded in which the dorsal digital nerves to the little and the ulnar side of the ring finger 

 were furnished by the musculo-spiral instead of by the ulnar and in which the inferior external 

 cutaneous branch extended to the first phalanx of the ring finger and the second phalanx of 

 the little finger. The radial nerve may supply the entire dorsum of the hand and the dorsal 

 aspect of all the fingers, or it may be absent, the musculo-cutaneous going to the thumb and 

 the ulnar to the remainder of the digits. The external division may send a branch to the 

 palm. The posterior interosseous may pass over the surface of the supinator brevis and may 

 furnish a branch to the anconeus muscle. Two instances are reported in which the posterior 

 interosseous supplied the opposed surfaces of the middle and index fingers. 



Practical Considerations. The musculo-spiral is more frequently paralyzed 

 than any of the other branches of the brachial plexus. Its axillary portion often 

 suffers from crutch pressure ; and the nerve is also particularly exposed to com- 

 pression where it passes between the triceps muscle and the humerus, as when the arm, 

 during sleep, is used for a pillow. It has been injured by violent contraction of the 

 triceps muscle, as in the act of throwing. It is frequently lacerated by the fragments 

 in fractures of the middle of the shaft of the humerus . When the lesion is in the axilla 

 the triceps will be included in the paralysis. If the portion in the arm is affected the tri- 

 ceps and anconeus will escape, but the following muscles will be paralyzed : the supina- 

 tors, the extensors of the hand, the extensor communis digitorum, together with the 

 extensor indicis, the extensor minimi digiti and the extensors of the thumb. The 

 characteristic symptom is the inability to extend the hand at the wrist (wrist drop), 

 and this is the most common form of musculo-spiral paralysis. 



THE THORACIC NERVES. 



The thoracic nerves (nn. thoracales) (Fig. 1105) consist of twelve pairs of sym- 

 metrical nerve-cords, the upper eleven of which, because of their position in the 

 intercostal spaces, are called intercostal nerves, and the twelfth, which lies below the 

 twelfth rib and is an occupant of the abdominal wall, the subcostal. Since only seven 

 ribs reach the sternum, the upper six thoracic nerves alone are continued throughout 

 their entire course in intercostal spaces. The lower six, with the exception of the 

 twelfth, after traversing their respective intercostal spaces proceed within the abdom- 

 inal wall, through which they course to within a short distance of the median line. 

 In accordance with the direction of the ribs, the upper nerves lie more horizontally 

 than the lower, the latter becoming more and more oblique as the lower part of the 

 abdominal wall is reached. As they advance from the spine, they distribute motor 

 filaments to the external and internal intercostals, the subcostals, the levatons 

 costarum, the serrati postici superior et inferior, the triangularis stcrni, the external 

 oblique, the internal oblique, the trausversalis, the reetus, the pyramidalis and a por- 

 tion of the diaphragm. Their cutaneous distribution comprises the integmm-nt 

 of the chest and abdomen anterior to the area supplied by the posterior primary 

 divisions of the thoracic nerves. On account of the presence of the shoulder girdle, 

 the usual nerve distribution is modified in the- upper thoracic region and the supra- 

 davicular branches of the rervieal plexus assume a function belonging to the thoracic 

 nerves. At the lower portion of the trunk the usual arrangement is likewise- alt en 



