568 THE NERVOUS SYSTEM. | | , 
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posterior and anterior primary divisions of the spinal nerves, presents primarily and 
essentially a segmental arrangement, masked and in some cases obliterated by develop- 
mental changes which have occurred in the parts supplied. In no region can a single 
nerve be traced to a complete segment. In the trunk between the limbs the nearest 
approach to a complete girdle is formed by such a nerve as the sixth thoracic nerve. 
In its cutaneous distribution it forms a perfect belt, the nerve by its posterior and 
anterior primary divisions supplying a distinctly segmental area from the middle line 
of the trunk behind to the sternum in front. Its muscular distribution, also, is 
almost perfectly segmental. The anterior primary division supplies, unaided, the 
intercostal muscles of the segment in which it lies. The posterior primary division 
supplies muscles in the back, not, however, in a strictly segmental manner, on account 
of the fact that the segmental myotomes have fused together in the back to give rise 
to complex longitudinal muscles, which are together supplied by the series of muscular 
branches derived from these posterior primary divisions. In other regions greater changes 
cause more marked deviations from a simple segmental type of distribution and give rise 
to the various plexuses, by which the trunk, and more particularly the limbs, are 
innervated. 
POSTERIOR PRIMARY DIVISIONS OF THE SPINAL NERVES. 
The posterior primary divisions of the spinal nerves are distributed generally to 
the skin of the back of the trunk, the back of the head, the shoulder and the 
buttock, and to the longitudinal muscles of the back, but not to the muscles of the 
linbs. 
Each posterior primary division divides as a rule into two parts, an internal and 
an external trunk (Hig. 421, p. 567). In the upper half of the body the internal 
trunks generally supply the cutaneous branches, while the external trunks are 
purely muscular nerves. In the lower part of the body the opposite is the case : 
the external trunks provide the cutaneous nerves and the internal trunks are 
custributed entirely to muscles. The cutaneous branches have a different course 
in the two cases. In the upper half of the back they course inwards and back- 
wards beneath and among the muscles to within a short distance of the spinous 
processes of the vertebrée, close to which they become superficial. They then 
extend outwards in the superficial fascia. In the lower half of the back the 
cutaneous nerves are directed downwards and outwards among the muscles, and 
become superficial at a greater distance from the middle line. 
CERVICAL NERVES. 
First Cervical Nerve (n. suboccipitalis)—It has already been pointed out that 
the dorsal root of this nerve may be rudimentary, or even absent altogether. Its 
posterior primary division is larger than the anterior primary division ; it does not 
divide into internal or external branches, and does not supply directly any 
cutaneous branch. 
Passing backwards in the space between the occipital bone and the posterior 
arch of the atlas, the nerve occupies the suboccipital triangle, and is placed below 
and behind the vertebral artery under cover of the complexus muscle. It supplies 
the following branches :— 
(a) Muscular branches to the complexus, recti capitis postici, major and minor, 
and obliqw, superior and inferior. 
(c) A communicating branch descends to join the second cervical nerve. 
The communicating branch may arise in common with the nerve to the obliquus inferior, and 
reach the second cervical nerve by piercing or passing over or beneath the obliquus inferior. Or 
it may accompany the nerve to the complexus, and communicate with the great occipital nerve, 
after piercing that muscle. 
Second Cervical Nerve.—The posterior primary division of this nerve is 
larger than the corresponding anterior primary division. It passes backwards in 
the interval between the atlas and axis, and below the obliquus inferior muscle. 
