690 THE NERVOUS SYSTEM. 



Third Cervical Nerve. This is much smaller than the second nerve. Near 

 its origin it forms a loop of communication with the second, and it may give off 

 a similar communicating branch to the fourth nerve. The main trunk divides 

 into medial cutaneous and lateral muscular branches. The lateral muscular 

 branch enters contiguous muscles ; the medial cutaneous branch passes backwards 

 and medially, and becomes superficial as the third occipital nerve (O.T. n. occipitalis 

 minimus), close to the median plane of the neck. It supplies fine branches to 

 the neck and scalp, and communicates with the greater occipital nerve. 



The fourth, fifth, and sixth cervical nerves are still smaller. Beneath the 

 semispinalis capitis each divides into lateral muscular and medial cutaneous 

 branches. The muscular branches supply neighbouring muscles ; the cutaneous 

 branches are small nerves, which, passing backwards, become superficial close to the 

 median plane. They supply the skin of the back of the neck. The sixth is the 

 smallest, and the cutaneous branches of the fifth and sixth nerves may be absent 

 altogether. In certain cases the fourth nerve forms, with the third, a loop of com- 

 munication from which muscles are supplied. 



Seventh and Eighth Cervical Nerves. These are the smallest of the posterior 

 rami of the cervical nerves. They give off ordinarily no cutaneous branches, 

 and end in the deep muscles of the back. There is occasionally a small cutaneous 

 offset from the eighth nerve. 



THORACIC NERVES. 



The posterior ramus of each thoracic nerve divides into a medial and a lateral 

 branch. In the case of the upper six or seven thoracic nerves the medial 

 branches are distributed chiefly as cutaneous nerves, only giving off small muscular 

 branches while the lateral branches are wholly muscular in their distribution ; 

 in the case of the lower five or 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 upper six or seven thoracic nerves innervate the skin 

 of the scapular region. The medial cutaneous branches, after a sinuous backward 

 course from their origin, among the dorsal muscles, reach the surface near the 

 spines of the vertebrse and are directed almost horizontally laterally over the 

 trapezius muscle. The first is small ; the second is very large and reaches to the 

 acromion. The rest diminish in size, from above downwards, and become more 

 and more oblique in direction. The lateral cutaneous branches of the lower five or 

 six thoracic nerves are directed from their origin obliquely downwards and laterally 

 among the parts of the sacro-spinalis muscle. Becoming cutaneous by piercing 

 the latissimus dorsi at some distance from the median plane, 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 latis- 

 simus dorsi muscle. 



LUMBAR NERVES. 



First three Lumbar Nerves. The posterior rami of the first three lumbar j 

 nerves subdivide into medial and lateral branches, in the same way as the lowers 

 thoracic nerves. The medial branches are muscular and innervate the deep! 

 muscles of the back. The lateral branches are chiefly cutaneous. They are| 

 directed obliquely downwards and laterally among the fibres of the sacro-spinalie| 

 and become superficial by piercing the lumbo-dorsal fascia, just above the iliac crest 

 and a short distance in front of the posterior superior iliac spine. They are ther 

 directed downwards in the superficial fascia of the buttock, and supply a length}! 

 strip of skin, extending from the median plane above the iliac crest to a pom i 

 distal to and behind the greater trochanter of the femur. There may be onbjji 

 two cutaneous branches, derived from the first two lumbar nerves; in other case 

 the three nerves are the branches of the twelfth thoracic and first two lumbaf'j 

 nerves. 



