160 THE NERVOUS SYSTEM 



THE INTERCOSTAL NERVES 



The Anterior Rami of the upper eleven Thoracic Nerves 

 form the intercostal nerves ; but the first intercostal is very 

 small and only supplies the first intercostal muscles. The 

 second, third, fourth, fifth and sixth not only supply the 

 intercostals but also give off lateral and anterior cutaneous 

 branches. The lateral cutaneous nerves pierce the deep fascia 

 near the mid-axillary line, and divide into anterior and 

 posterior branches, which supply the skin over the lateral 

 aspect of the body (Fig. 69). The lateral branch of the second 

 intercostal nerve, however, supplies the skin on the postero- 

 medial aspect of the arm and is termed the intercosto-brachial 

 nerve. Near the sternum, the intercostal nerves turn forwards 

 and their terminal branches constitute the anterior cutaneous 

 nerves. 



In consequence of the areas supplied by the second inter- 

 costal nerve, it is not surprising to find that, in the condition 

 of " thoracic-ulnar analgesia," which is frequently an early sign 

 of tabes dorsalis, there is loss of sensibility over the upper 

 part of the chest and the medial side of the arm. 



It must be remembered that although the intercostal nerves 

 follow a very oblique course as they pass round the body, 

 their branches descend all to the same level before supplying the 

 skin. Each intercostal nerve is responsible for the supply of a 

 horizontal band of skin, which corresponds in level to the 

 terminal twigs of its cutaneous branches. 



The lower five intercostal nerves and the subcostal nerve 

 correspond to the upper intercostal nerves, except that their 

 terminal branches extend beyond the costal margin and gain 

 the anterior abdominal wall. On this account, the anterior 

 abdominal wall is often the site of referred pain in cases of 

 pneumonia and pleurisy. These nerves give off lateral and 

 anterior cutaneous branches, but, in addition to supplying the 

 intercostal muscles, they innervate the rectus abdominis, the 

 internal and external obliques and the transversus muscle. 



