Page:EB1911 - Volume 19.djvu/420

 branches which have a varicose appearance, form a rich plexus, and end in knobs. These organs are found between the true skin and subcutaneous tissue of the fingers.

6. Organs of Golgi are found in tendons. Nerve fibres penetrate the tendon bundles and divide in a tree-like manner to end in little disks and varicosities.

7. Neuro-muscular spindles are small fusiform bundles of embryonic muscle fibres among which the nerve fibres end by encircling them and forming flattened disks. These are sensory endings, and must not be confused with the motor end plates. They are found in most of the striped muscles of the body.

Motor nerves end in striped muscle by motor end plates. These are formed by a nerve fibre approaching a muscle fibre and suddenly losing its myelin sheath while its neurilemma becomes continuous with the sarcolemma of the muscle fibre. The axis cylinder divides, and its ramifications are surrounded by a disk of granular matter containing many clear nuclei. In very long muscle fibres more than one of these end plates are sometimes found. Involuntary motor endings are usually found in sympathetic nerves going to unstriped muscle. The fibres form minute plexuses, at the points of union of which small triangular ganglion cells are found. After this the separate fibrils of the nerve divide, and each ends opposite the nucleus of an unstriped muscle cell.

This system is made up of two gangliated cords running down one on each side of the vertebral column and ending below in the median coccygeal ganglion (g. impar). In the neck the cords lie in front of the anterior tubercles of the transverse processes of the cervical vertebrae, in the thorax, in front of the heads of the ribs, while in the abdomen they lie in front of the sides of the bodies of the

vertebrae. In addition to these cords there are numerous ganglia and plexuses through which the sympathetic nerves pass on their way to or from the viscera and blood-vessels. A typical ganglion of the sympathetic chain is connected with its corresponding spinal nerve by two branches called rami communicantes, one of which is grey and the other white (see fig. 4). The white consists of medullated fibres belonging to the central nervous system, and these are splanchnic afferent or centripetal, and efferent or centrifugal. The efferent fibres lie in the anterior roots of the spinal nerves, and, like all the fibres there, are either motor or secretory. They are the motor paths for the unstriped muscle of the vessels and viscera, and the secretory paths for the cells of the viscera. In the course of each fibre from the nerve cell in the spinal cord, of which it is an axon, to the vessel or viscus it supplies, there is always a break where it arborizes round a ganglion cell, and this may be in its own ganglion of the sympathetic chain, in a neighbouring ganglion above or below, or in one of the so-called collateral ganglia interposed between the sympathetic chain and the viscera. In addition to these there are a certain number of vaso-dilator and viscero-inhibitory fibres, which run without any cell connexions from the spinal or cranial nerve to the viscera. The splanchnic afferent or centripetal fibres are the sensory nerves from the viscera, and have no cell connexions until they reach the spinal ganglia on the posterior roots of the spinal nerves, which they do by traversing the gangliated cord of the sympathetic. The fibres of the white rami communicantes are remarkable for their small diameter, and the efferent fibres, at all events, are only found in two regions, one of which is called the thoracico-lumbar stream and extends from the first or second thoracic to the second or third lumbar nerve, while the pelvic stream is found from the second to the fourth sacral nerves. The grey rami communicantes are found in connexion with all the spinal nerves, though they are irregular in the paths by which they reach the sympathetic ganglia from the cells of which they spring; their fibres are mainly non-medullated, and pass into both roots of the spinal nerves and also into the anterior and posterior primary divisions of those nerves. In this way they reach the body wall and limbs, and are somatic vaso-motor secretory and pilo-motor fibres, supplying the vessels, glands and hair muscles of the skin and its glands. The sympathetic ganglia, from which these nerves come, contain multipolar nerve cells with one axon and several dendrites as well as a number of medullated fibres passing through, and much connective tissue.