THE DISTRIBUTION OF NERVES.
The distribution of the cranial nerves has been already indicated. Each spinal nerve, after it has issued from its opening in the back bone, splits into two divisions, one of which pro ceeds to supply parts behind the spine, while the other passes forward towards the front. The first eight spinal nerves on each side are called cervical, the next twelve are dorsal, the next five lumbar, then five sacral,and one coccygeal. The front divisions of the first four cervical unite to form what is called the cervical plexus, from which branches are supplied to the muscles of the neck. One very important nerve spring ing from the group is called the phrenic, which passes down the front of the chest to supply the diaphragm or midriff, upon the movements of which breathing so much depends. The other four of the cervical nerves and one of the dorsal unite to form the brachial plexus, from which nerves proceed to the upper limb. They enter the arm from the arm-pit, which they cross in company with the large blood-vessels for the limb. One of them winds round a groove in the upper arm-bone and passes to the muscles of the back of the arm. It is called the mus culo-spiral nerve. A second, the ulnae nerve, descends on the inner side of the arm. At the elbow it rests in an interval between the inner projecting process of the arm-bone and the point of the elbow. When the elbow is knocked at this place the shock given to the nerve produces the sensation ascribed to " knocking the funny bone". The nerve passes to supply the little finger and the neighbouring side of the ring finger. If these two fingers become numb, therefore, it is due to paralysis of this nerve. The remaining fingers and the thumb side of the ring-finger are supplied by the median nerve, which passes down the middle of the arm. All these nerves are mixed, that is, they confer power of motion and sensation.
Of the twelve dorsal nerves the posterior divisions supply the muscles and skin of the back, the anterior divisions supply branches, called intercostal nerves, to the spaces between the ribs.
The five lumbar nerves also send their pos terior branches to time back ; while the anterior divisions of the first four unite to form a group called the lumbar plexus. From it branches pass to the belly and genitals, and large branches to the muscles of the front of the thigh, and to the skin of the front and inner side of the thigh, and to the skin of the inner side of the leg and foot.
There is also a sacral plexus, formed by the anterior divisions of the sacral nerves, reinforced by the last lumbar.
From this group proceeds the largest nerve in the body, the sciatic nerve --- a neuralgic affection of which is called sciatica. It gives branches to the muscles at the back of the thigh, and branches also to muscles below the knee, and to the skin of the leg and foot. The nerve escapes from the pelvis just outside of the seat bone, and passes right down the middle of time back of the thigh till the back of the knee-joint is reached, where it divides into two branches called internal and external popliteal. The first of these branches continues the main course down the back of the leg, where it is called posterior tibial. It divides on the inner side of the heel into two branches, which between them supply the sole of the foot and the sides of the toes. The external branch turns round below the knee to the front of the outer side of the leg, down which it passes as the anterior tibial, supplying muscles on the way, and ends in branches to the skill of the back of the foot.