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 kinaesthetic 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.