572 THE NERVOUS SYSTEM. 
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supply the multifidus spine: muscle ; others, piercing the great sacro-sciatic ligament, form 
secondary loops beneath the gluteus maximus muscle. From the secondary loops, two or 
more cutaneous branches arise, which, after traversing the muscle, supply the skin of the 
buttock. 
Posterior Sacro-coccygeal Nerve.—The posterior divisions of the fourth and 
fifth sacral nerves do not divide into internal and external branches. They unite — 
together, and, descending behind the coccyx, receive the minute posterior primary 
division of the cocc cygeal nerve. The union of the three nerves constitutes the 
posterior sacro-coccygeal nerve, which, after perforating the sacro-sciatic ligament, 
is distributed to the skin in the neighbourhood of the coccyx. It supplies no 
muscles. This nerve is the representative of the superior caudal trunk of tailed 
animals. 
MoRPHOLOGY OF THE POSTERIOR PRIMARY DIVISIONS. 
There are several points of morphological interest in relation to the posterior primary 
divisions of the spinal nerves. 
1. Muscular Distribution.—In their muscular distribution they are strictly limited to the 
longitudinal muscles of the back : namely, those associated with the axial skeleton. 
2. Cutaneous Distribution.—Their cutaneous distribution represents two points of interest. 
A. In the first place, while the skin of the back is supplied in a regularly segmental manner 
by the several nerves, certain of them fail to reach the surface to become cutaneous. The absence 
of a cutaneous branch from the suboecipital nerve may be due either to the absence of a perfect 
dorsal root or to its communication with the second nerve. The other nerves which do not 
usually supply the skin are the seventh and eighth cervical, and the fourth and fifth lumbar 
nerves. These nerves are placed in the centre of regions in which the upper and lower limbs are 
developed. They are minute nerves, while the corz responding anterior primary divisions are among 
the largest of the spinal nerves. Thus opposite the centre of each limb, posteriorly, there is a hiatus 
in the ‘segmental distribution of the posterior primary divisions of the spinal nerves to the skin 
of the shoulder and buttock, associated with the formation of the limbs, and the extension into 
them of the greater part of the nerves of the region. This gap, in the case of the upper limb, 
commences at the level of the vertebra prominens ; in the case of the lower limb it commences 
opposite the level of the posterior superior iliac spine. It can be continued on to each limb as 
a hypothetical area (the dorsal axial line), which indicates the area of contact (and over- 
lapping) of cutaneous nerves not in strictly numerical sequence. Thus, in the region of the 
shoulder, the sixth (or fifth) cervical nerve innervates an area of skin adjoining that supplied by 
the first thoracic nerve; in the region of the buttock the third lumbar nerve supples an area 
contiguous with that supplied by the first sacral nerve. 
B. The cutaneous branches of the posterior primary divisions of the spinal nerves differ from 
the muscular branches in their penetration into regions beyond those suppled by their motor 
roots. The cutaneous branches, in regions where outgrowth or extension from the trunk has 
oecurred, follow this outgrowth ; and, im consequence, supply skin covering parts which do not 
belong to segments represented by the nerves in question. Thus the second and third cervical 
nerves (great and least occipital) are drawn upwards so as to supply the posterior part of the 
scalp ; the upper thoracic nerves are drawn outwards over the scapular region ; the upper lumbar 
and sacral nerves supply the skin of the buttock; and the sacro-coccygeal nerve forms a rudimentary 
caudal nerve. 
3. Plexuses.— The plexuses formed by the posterior primary divisions of the upper 
cervical and upper sacral nerves are the simplest met with in the human body. The 
posterior cervical plexus is one from which muscular branches are supplied; the posterior 
sacral plexus is mainly concerned in producing cutaneous offsets. In the case of the posterior 
cervical plexus the loops of communication between the first three or four cervical nerves result 
in the formation of a series of nerves for the supply of the semi-spinalis, complexus, and other 
muscles, which bring into contact with these muscles, simultaneously, a considerable area of the 
spinal cord, and provide a combined and simultaneous innervation for the several parts of each 
muscle, which has thus an extensive source of energy in the spinal cord. In the case of the 
posterior sacral plexus, the formation of loops between the nerves results in the innervation of 
any given spot in the cutaneous area supplied from these loops by more than one spinal nerve. 
As has been said already, the cutaneous nerves, even without the formation of plexuses, overlap 
in their cutaneous distribution. The formation of a plexus causes a more intimate union of 
neighbouring spinal nerves, so that stimulation of the surface affects a wider area in the spinal 
cord than if the nerves passed separately to the surface. While segmentation becomes less 
obvious, inereased co-ordination is effected both of movement and sensation. 
ANTERIOR PRIMARY DIVISIONS OF THE SPINAL NERVES. 
The anterior primary divisions of the spinal nerves are, with the exception of 
the first two cervical nerves, much larger than the corresponding posterior primary 
divisions. Composed of elements of both dorsal and ventral roots, each nerve 
