SACRAL AND COCCYGEAL NERVES. 691 



The fourth and fifth lumbar nerves (like the last two cervical nerves) usually 

 supply only muscular branches to the longitudinal muscles of the back. The fifth 

 i nerve in many cases sends a branch to form a loop of connexion with the posterior 

 : ramus of the first sacral nerve (posterior sacral plexus). 



SACRAL AND COCCYGEAL NERVES. 



The posterior rami of the sacral nerves issue from the posterior sacral foramina. 

 As in the case of the thoracic and lumbar nerves, the upper sacral nerves differ 

 from the lower in their distribution. 



The first three sacral nerves supply medial muscular branches for the 



multifidus, and lateral cutaneous branches which pierce the fibres of the sacro- 



1 tuberous ligament and the glutseus maximus muscle, and supply the skin over 



J the back of the sacrum and contiguous part of the buttock, giving rise to the 



posterior sacral plexus. 



The posterior sacral plexus consists, like the posterior cervical plexus, of loops 

 or plexiform communications over the back of the sacrum between the posterior rami of 

 the first three sacral nerves, to which are frequently joined branches of the last lumbar 



j nerve and fourth and even the fifth sacral nerve. From these loops branches proceed to 

 supply the multifidus muscle ; others, piercing the sacro-tuberous ligament, form secondary 

 loops beneath the glutseus maximus muscle. From the secondary loops, two or more 



I cutaneous branches arise, which, after traversing the muscle, supply the skin over the 



\ sacrum and medial part of the buttock. 



Posterior Ano-coccygeal Nerve. The posterior rami of the fourth and fifth 

 sacral nerves do not divide into medial and lateral branches. They unite together 

 to form a loop which is joined by the minute posterior ramus of the coccygeal 

 nerve. The union of the three nerves constitutes the posterior ano-coccygeal nerve, 

 which, after perforating the sacro-tuberous 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 RAMI. 



There are several points of morphological interest in relation to the posterior rami of 

 i 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 alone. 



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 at alL The absence of a cutaneous 

 branch from the sub-occipital nerve may be due either to the absence of a perfect posterior root, or 

 to its communication with the second nerve. The other nerves which do not usually supply the 

 skin are the last two, three, or four 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 corresponding anterior rami 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 rami of the spinal nerves to the skin of the shoulder 

 and buttock, attributable to 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 overlapping) 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 eighth cervical 



or first thoracic nerve ; in the region of the buttock the third, lumbar nerve supplies an area 



i contiguous with that supplied by the fifth lumbar or first sacral nerve. 



. The cutaneous- branches of the posterior rami of the spinal nerves differ from the muscular 

 r i branches in respect of their penetration into regions beyond those supplied by their motor 



' roots. The cutaneous branches, in regions where outgrowths or extensions from the trunk have 

 occurred, follow these extensions ; and, in consequence, supply skin covering parts which do not 

 belong to segments represented by the nerves in question. Thus, the second and third cervical 

 nerves (greater and third occipital) are drawn upwards so as to supply the posterior part of the 

 scalp ; the upper thoracic nerves are drawn laterally over the scapular region ; the upper lumbar 

 and sacral nerves supply the skin of the buttock ; and the ano-coccygeal nerve forms a rudi- 

 mentary caudal nerve. 



