rOSTERIOR DIVISIONS OF SPINAL NERVES. 571 



spinal cord beino- nearly opposite the foramina by which they respec- 

 tively leave the canal, tliese roots are comparatively short. But the 

 distance between the two points referred to is gradually augmented 

 from nerve to nerve downwards, so that the place of origin of the lower 

 cervical nerves is the breadth of at least one vertebra, and that of the 

 lower dorsal nerves about the breadth of two vertcbraj above the fora- 

 mina by which they respectively emerge from the canal. Moreover, as 

 the spinal cord extends no farther than the first lumbar vertebra, the 

 length of the roots of the lumbar, sacral and coccygeal nerves increases 

 rapidly from nerve to nerve, and in each case may be estimated by the 

 distance of the foramen of exit from the extremity of the cord. Owing 

 to their length, and the appearance they present in connection with the 

 spinal cord, the aggregation of the roots of the nerves last referred to 

 has been named the " cauda equina.' 



The direction the roots take within the canal requires brief notice. 

 The first cervical nerve is directed horizontally outwards. The roots 

 of the lower cervical and dorsal nerves at fivst descend over the spinal 

 cord, held in contact with it by the arachnoid, till they arrive opposite 

 the several intervertebral foramina, where they are directed horizontally 

 outwards. The nerves of the cauda equina run in the direction of the 

 spinal canal. 



Division of the nerves. — The two roots of each of the spinal 

 nerves unite immediately beyond the ganglion, and the trunk thus 

 formed separates, as already mentioned, into two divisions, an anterior 

 and a posterior, which are called primary branches or divisions. 



In the detailed description of the spinal nerves which follows, we shall 

 begin with their posterior primary divisions, calling attention first to 

 certain characters common to the whole of them, and afterwards stating 

 separately the arrangement peculiar to each group of nerves (cervical, 

 dorsal, &c.) 



POSTERIOR PRIMARY DIVISIOI-TS OF THE SPINAL 



NERVES. 



The posterior divisions of the spinal nerves are, with few exceptions, 

 smaller than those given to the fore part of the body. Springing from 

 the trunk which results from the union of the roots of the nerve in the 

 intervertebral foramen, or frequently by separate fasciculi from each of 

 the roots, eacli turns backwards at once, and soon divides into two 

 parts, distinguished as external and internal, distributed to the muscles 

 and the integument behind the spine. The first cervical, the fourth and 

 fifth sacral and the coccygeal nerve are the only nerves the posterior 

 divisions of which do not separate into external and internal branches. 



The Sub-occipital Nerve. — The posterior division of the sub-occi- 

 pital nerve, which is the larger of the two primary divisions, emerging 

 over the arch of the atlas, between this and the vertebral artery, enters 

 the space bounded by the larger rectus and the two oblique muscles, 

 and divides into branches for the surrounding muscles. 



a. One branch descends to the lower oblique muscle and gives a filament, 

 through or over the fibres of that muscle, to join the second cervical nerve. 



b. Another ascends over the larger rectus muscle, supplying it and the smaller 

 rectus. 



c. A third enters the upjier oblique muscle. 



d. A fourth sinks into the complexus, where that muscle covers the nerve and 

 its branches. 



