CHAPTER XVI
THE NERVES

Anatomy.—A nerve-trunk is made up of a variable number of bundles of nerve fibres surrounded and supported by a framework of connective tissue. The nerve fibres are chiefly of the medullated type, and they run without interruption from a nerve cell or neuron in the brain or spinal medulla to their peripheral terminations in muscle, skin, and secretory glands.

Each nerve fibre consists of a number of nerve fibrils collected into a central bundle—the axis cylinder—which is surrounded by an envelope, the neurolemma or sheath of Schwann. Between the neurolemma and the axis cylinder is the medullated sheath, composed of a fatty substance known as myelin. This medullated sheath is interrupted at the nodes of Ranvier, and in each internode is a nucleus lying between the myelin and the neurolemma. The axis cylinder is the essential conducting structure of the nerve, while the neurolemma and the myelin act as insulating agents. The axis cylinder depends for its nutrition on the central neuron with which it is connected, and from which it originally developed, and it degenerates if it is separated from its neuron.

The connective-tissue framework of a nerve-trunk consists of the perineurium, or general sheath, which surrounds all the bundles; the epineurium, surrounding individual groups of bundles; and the endoneurium, a delicate connective tissue separating the individual nerve fibres. The blood vessels and lymphatics run in these connective-tissue sheaths.

According to Head and his co-workers, Sherren and Rivers, the afferent fibres in the peripheral nerves can be divided into three systems:—

  1. Those which subserve deep sensibility and conduct the impulses produced by pressure as well as those which enable the patient to recognise the position of a joint on passive movement (joint-sensation), and the kinęsthetic sense, which recognises that active contraction of the muscle is taking place (active muscle-sensation). The fibres of this system run with the motor nerves, and pass to muscles, tendons, and joints. Even division of both the ulnar and the median nerves above the wrist produces little loss of deep sensibility, unless the tendons are also cut through. The failure to recognise this form of sensibility has been largely responsible for the conflicting statements as to the sensory phenomena following operations for the repair of divided nerves.

  2. Those which subserve protopathic sensibility—that is, are capable of responding to painful cutaneous stimuli and to the extremes of heat and cold. These also endow the hairs with sensibility to pain. They are the first to regenerate after division.

  3. Those which subserve epicritic sensibility, the most highly specialised, capable of appreciating light touch, e.g. with a wisp of cotton wool, as a well-localised sensation, and the finer grades of temperature, called cool and warm (72°–104° F.), and of discriminating as separate the points of a pair of compasses 2 cms. apart. These are the last to regenerate.

A nerve also exerts a trophic influence on the tissues in which it is distributed.

The researches of Stoffel on the minute anatomy of the larger nerves, and the disposition in them of the bundles of nerve fibres supplying different groups of muscles, have opened up what promises to be a fruitful field of clinical investigation and therapeutics. He has shown that in the larger nerve-trunks the nerve bundles for special groups of muscles are not, as was formerly supposed, arranged irregularly and fortuitously, but that on the contrary the nerve fibres to a particular group of muscles have a typical and practically constant position within the nerve.

In the large nerve-trunks of the limbs he has worked out the exact position of the bundles for the various groups of muscles, so that in a cross section of a particular nerve the component bundles can be labelled as confidently and accurately as can be the cortical areas in the brain. In the living subject, by using a fine needle-like electrode and a very weak galvanic current, he has been able to differentiate the nerve bundles for the various groups of muscles. In several cases of spastic paralysis he succeeded in picking out in the nerve-trunk of the affected limb the nerve bundles supplying the spastic muscles, and, by resecting portions of them, in relieving the spasm. In a case of spastic contracture of the pronator muscles of the forearm, for example, an incision is made along the line of the median nerve above the bend of the elbow. At the lateral side of the median nerve, where it lies in contact with the biceps muscle, is situated a well-defined and easily isolated bundle of fibres which supplies the pronator teres, the flexor carpi radialis, and the palmaris longus muscles. On incising the sheath of the nerve this bundle can be readily dissected up and its identity confirmed by stimulating it with a very weak galvanic current. An inch or more of the bundle is then resected.