162 THE VOYAGE OF H.M.S. CHALLENGER. 



DoESAL Nerves. 



Intercostal Nerves. — The anterior branches of the dorsal nerves are nine in number. 

 Of these the first emerges from the intervertebral foramen corresponding to the 

 intercostal space, which is bounded in front by the first or rudimental, and behind by the 

 second, while the ninth nerve lies behind the ninth vertebral rib. The anterior division 

 of the first dorsal nerve unites with those of the last two cervical nerves to form the large 

 cord of the brachial plexus, along with which it is more particularly described. It 

 furnishes no branch to the first intercostal space. 



All the other intercostal nerves, with the exception of the last, pass outwards, lying on 

 the internal intercostal muscles as far as the middle in length of the vertebral segments 

 of the ribs. Here they pierce the intercostal muscles, and extending downwards under 

 cover of the skin, terminate by supplying branches to the lateral and inferior thoracic 

 waEs, nearly as far as the inferior middle line of the body. The intercostal nerves, there- 

 fore, do not lie between the sternal segments of the ribs, but escape from the intercostal 

 spaces before reaching them. As the nerves extend downwards they give branches to 

 the intercostal muscles, as well as to the muscles of the abdominal wall. 



The second intercostal nerve furnishes a branch to the brachial plexus. 



The anterior branch of the last dorsal nerve extends downwards behind the ninth 

 vertebral rib, and, after supplying a branch to the obliquus intemus abdominis, 

 terminates by being distributed to the other muscles of the abdominal wall. None of 

 the intercostals give ofi" lateral cutaneous branches as in the mammal, the place of these 

 being taken by irregular branches supphed by the intercostal nerves, after they have 

 escaped from the intercostal spaces. 



LuMBO- Sacral Nerves. 



To avoid ambiguity, I include all the spinal nerves between the last dorsal and the 

 first coccygeal in the category of lumbo-sacral nerves. Although not forgetful of the 

 work of Gegenbaur and others in the determination of the exact limitation of the 

 lumbar, sacral, and coccygeal portions of the avian vertebral column, yet so many 

 difficulties are met with in regard to this limitation, whether the attempt be founded 

 on a consideration of the arrangement of the spinal nerves, or on that of the relation 

 which a variable number of vertebrae in difi"erent species bear to the pelvic bones, that it 

 appears to me that it will conduce to the intelligibility of my description if I simply 

 include the spinal nerves lying between the last dorsal vertebra and the first moveable 

 coccygeal under the name of lumbo-sacral. By adopting this plan, first suggested by 

 Mivart,^ I shall avoid the adoption of any morphological views which, while they may 



^ Trans. Zool. Soc, vol. viii. pt. 7, 1874. 



