DISSECTION OF THE BACK AND THORAX. 139 



in the lumbar region, forming a second swelling — the lumbar enlarge- 

 ment, from which the nerves for the supply of the hind limb are de- 

 tached. Beyond the lumbar enlargement the cord rapidly becomes 

 reduced in volume, and tapers to a point about the 2nd sacral seg- 

 ment. This tapering extremity of the cord — the eonus medullaris — is 

 prolonged backwards by the filum terminale, into which its nervous 

 structure is continued for a little distance. 



The Spinal Nerves of the horse number forty-two or forty-three pairs, 

 and their number in the diflFerent regions of the vertebral column is 

 expressed in the following formula : — 



Cs-DisLeSjCj or 6- 



The 1st cervical nerve leaves the canal by the antero-internal foramen 

 of the atlas, the 2nd by the foramen in the front of the arch of 

 the axis, and the others in succession pass out by the intervertebral 

 foramina. 



In the other regions the nerves are numbered according to the 

 vertebrse behind which they emerge ; thus, the 1st dorsal nerve emerges 

 by the intervertebral foramen behind the 1st dorsal vertebra, and so on 

 with the others. 



In the cervical region the nerves pass nearly directly outwards from 

 the cord to their points of exit from the canal. In the dorsal region, 

 however, it will be observed that each nerve is slightly inclined back- 

 wards from the side of the cord to the foramen by which it emerges. In 

 the lumbar region this backward inclination of the nerves becomes 



Fig. 16. 



Portion oi' Spinal Cord with the Boots of the Nerves (Quuin). 

 1 Inferior median flBsure ; 2. Superior median fissure ; S. Infero-lateral fissure (exaggerated); 

 4 s'uDero-lateral fissure ; 6. Inferior roots, passing under the ganglion (on the left side these are cut) ; 

 e! Superior roots, the fibres of which pass into the ganglion— 6' ; 7. The united or compound nerve ; 

 S. Superior primary branch ; 9. Inferior primary branch. 



augmented, and it continues to increase in the same way in each nerve 

 of the sacral and coccygeal regions. The sacral nerves thus have their 

 roots detached from the lumbar part of the cord, while the coccygeal 

 nerves are given off by the terminal part of the cord, which, as already 



