632 THE SPINAL NERVES. 



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 DIVISIONS 



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 inter- 

 vertebral foramen, or frequently by separate fasciculi from each of the 

 roots, each turns backwards at once, and soon divides into two parts, distin- 

 guished as external and internal, distributed to the muscles and the integu- 

 ment behind the spine. The first cervical, the fourth aud fifth sacral and 

 the coccygeal nerve are the only nerves the posterior divisions of which do 

 not separate into external and internal branches. 



THE SUBOCCIPITAL NERVE. The posterior division of the suboccipital 

 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. 



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

 rectus. 



c. A third enters the upper oblique muscle. 



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

 branches. 



A cutaneous branch is occasionally given to the back of the head ; it accompanies 

 the occipital artery, and is connected beneath the integument with the great and 

 small occipital nerves. 



Fig. 422. SUPERFICIAL AND DEEP DISTRIBUTION OP THE POSTERIOR PRIMARY DIVISIONS 

 OF THE SPINAL NERVES (from Hirschfeld and Leveille). 



On the left side the cutaneous branches are represented as lying upon the superficial 

 layer of muscles ; on the right side, the superficial muscles having been removed, the 

 splenius and complexus have been divided in the neck, and the erector spinae separated 

 and partially removed in the back, so as to expose the deep issue of the nerves. 



, a, lesser occipital nerve from the cervical plexus ; 1, external muscular branches of 

 the first cervical nerve and union by a loop with the second ; 2, placed on the rectus 

 capitis posticus major, marks the great occipital nerve passing round the short muscles 

 and piercing the complexus : the external branch is seen to the outside ; 2', cranial dis- 

 tribution of the great occipital ; 3, external branch of the posterior primary division of 

 the third nerve ; 3', its internal branch, or third occipital nerve ; 4', 5', 6', 7', 8', 

 internal branches of the several corresponding nerves on the left side : the external 

 branches of these nerves proceeding to muscles are displayed on the right side : dl, io 

 d 6, and thence to c? 12, external muscular branches of the posterior primary divisions of 

 the twelve dorsal nerves on the right side ; d 1', to d 6', the internal cutaneous branches 

 of the six upper dorsal nerves on the left side ; d 7', to d 12', cutaneous branches of the 

 six lower dorsal nerves from the external branches ; Z, I, external branches of the pos- 

 terior primary branches of several lumbar nerves on the right side piercing the muscles, 

 the lower descending over the gluteai region ; I', l\ the same more superficially on the left 

 side ; s, s, on the right side, the issue and union by loops of the posterior primary divisions 

 of four sacral nerves ; s' t s', some of these distributed to the skin on the left side. 



