Third cervical nerve is called the least occipital nerve, and is rather small, but its distribution is similar to that of the posterior divisions of the other spinal nerves.

The anterior divisions of the rest supply the trunk and the limbs. They form what are called plexuses, which are groups of nerves joined up together. There are five plexuses-viz.:

Cervical plexus, formed by cervical nerves 1, 2, 3, and 4, supplies the muscles and skin of the neck.

Brachial plexus, formed by cervical nerves 5, 6, 7, 8, and a branch of the first dorsal nerve, supplies the upper limb.

Lumbar plexus, formed by a branch of the twelfth dorsal nerve, lumbar nerves 1, 2, 3, and a branch of the fourth lumbar nerve, supplies the anterior, inner, and outer surfaces of the thigh.

Sacral plexus, formed by lumbar nerves 4 and 5, and sacral nerves 1, 2, 3, supplies buttock, posterior surface of thigh and leg and foot.

Pudendal plexus, formed by a branch of the third sacral nerve, sacral nerves 4 and 5, and the coccygeal nerve, supplies the perineum.

The anterior divisions of the twelve dorsal or thoracic nerves supply the skin and muscles on the anterior and lateral surfaces of the trunk. The first eleven are intercostal, the twelfth lies below the last rib.

The Intercostal Nerves are much alike in their course and distribution, but some of them differ slightly from the others.

A typical thoracic nerve enters the posterior end of the subcostal groove, and lies between the intercostal muscles. Coursing forwards, it pierces the internal intercostal muscle about the middle of the chest wall, and lies on the pleura. Near the middle line it pierces again the internal intercostal muscle and the aponeurosis of the external intercostal, and supplies the skin over the front of the chest corresponding to the anterior half of the space to which it belongs.

Branches

Cutaneous, a lateral cutaneous branch is given off in the mid-axillary line to the skin over the space to which it belongs; it divides into anterior and posterior branches.

Muscular, to the intercostal muscles.

First thoracic nerve divides into two parts. The upper larger part passes over the neck of the first rib, enters the neck behind the subclavian artery, and joins with the other nerves forming the brachial plexus. The lower part enters the subcostal groove and supplies the intercostal muscles, but as a rule has no cutaneous branches.

Fig. 62. - Cutaneous Nerves of Trunk.

Second thoracic nerve has the same course as a typical thoracic nerve, but the lateral cutaneous branch, called the intercostohumeral nerve, is of unusually large size. It crosses the axilla and supplies the skin of the armpit, and the inner side of the posterior surface of the arm as far as the elbow.

Third thoracic nerve is a typical one, except that the posterior half of the lateral cutaneous branch also extends into the arm, and supplies a small portion of the posterior surface of the root of the limb.

Fourth, fifth, and sixth thoracic nerves are typical.

Seventh, eighth, ninth, tenth, and eleventh thoracic nerves at the anterior ends of their intercostal spaces pierce the attachment of the diaphragm and the transversalis abdominis, and pass forwards between the latter and obliquus internus. Their anterior ends become cutaneous by piercing the sheath of the rectus and the muscle itself. They supply the intercostal muscles of their own spaces and the abdominal muscles, and help to supply the diaphragm. Their cutaneous branches are similar to those of the typical nerves.

Twelfth thoracic nerve passes downwards under psoas and then passes forwards, similarly to those just above it. It gives off a large lateral cutaneous branch, which, passing down between the abdominal muscles, becomes cutaneous just above the iliac crest, and supplies the skin of the buttock as far down as the great trochanter of the femur.