SACRAL AND COCCYGEAL NERVES. 571 
cutaneous branch is minute, and may be absent altogether. In certain cases the 
fourth nerve forms, with the third, a loop of communication from which muscles are 
supplied. 
Seventh and Highth Cervical Nerves.-—These are the smallest of the posterior 
primary divisions of the cervical nerves. They give off ordinarily no cutaneous 
branches, and end in the deep muscles of the back. 
THORACIC NERVES. 
The posterior primary division of each thoracic nerve divides into an internal 
and external branch. In the case of the upper six thoracic nerves the internal 
branches are chiefly distributed as cutaneous nerves, while the external branches 
are wholly muscular in their distribution; in the case of the lower six thoracic 
nerves the opposite is the case. In all cases the muscular branches serve to 
innervate the longitudinal muscles of the back. The distribution of the cutaneous 
branches is different in the upper and lower part of the back. The wpper six or 
seven thoracic nerves innervate the skin of the scapular region. The internal 
cutaneous branches, after passing backwards from their origin among the dorsal 
muscles, reach the surface near the spines of the vertebre and are directed almost 
horizontally outwards over the vertebral border of the scapula. The first is small ; 
the second is very large and reaches to the acromion process. The rest diminish in 
size, from above downwards. The external cutaneous branches of the lower jive or 
sia thoracic nerves are directed from their origin obliquely downwards and outwards 
among the parts of the erector spine muscle. Becoming cutaneous by piercing the 
latissimus dorsi at some distance from the middle line they supply the skin of the 
back in the lower part of the chest and loin, the lowest nerves (eleventh and 
twelfth) reaching over the iliac crest on to the buttock. The lower nerves often 
subdivide into two branches before or after their emergence from the latissimus 
dorsi muscle. 
LUMBAR NERVES. 
First three Lumbar Nerves.—The posterior primary divisions of the first 
three lumbar nerves subdivide into internal and external branches, similar in 
their general arrangement to the lower thoracic nerves. The internal branches 
are muscular and innervate the deep muscles of the back. The external branches 
are chiefly cutaneous. They are directed obliquely downwards and outwards 
among the fibres of the erector spine and become superficial by piercing the 
vertebral aponeurosis, Just above the ihac crest and a short distance in front of 
the posterior iliac spine. They are then directed downwards in the superficial 
fascia of the buttock and supply a lengthy strip of skin, extending from the middle 
line above the ihae crest to a point below and behind the ereat trochanter of the 
femur. 
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 
nerve sends a communicating branch in many cases to form a loop with the 
posterior primary division of the first sacral nerve (posterior sacral plexus). 
SACRAL AND COCCYGEAL NERVES. 
The posterior primary divisions 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 internal muscular branches for the 
multifidus spine, and external cutaneous branches which pierce the fibres of the 
sacro-sciatic ligament and the gluteus maximus muscle, and supply the skin over 
the back of the sacrum and contiguous part of the buttock. 
The posterior sacral plexus consists, like the posterior cervic sal plexus, of loops 
or plexiform communications over the back of the sacrum between the posterior primary 
divisions of the first three sacral nerves, to which are frequently joined branches of the 
last lumbar nerve and fourth sacral nerve. From these loops branches proceed to 
