Part 80

The ganglionic portion, or gray matter, is arranged in the center of the cord in two comma-shaped parts forming an X. The white matter, or the material forming the fibers, is arranged around the central ganglionic portion.

The spinal cord may be divided into two lateral symmetrical halves. There are two longitudinal fissures, one on the upper and the other on the lower half. The upper called the superior median fissure is narrow but deep. The one on the inferior side, called the inferior median, is usually more pronounced. The superior parts of the gray matter are called the superior cornua, or horns; the inferior parts the inferior cornua, or horns. The center of the cord is pierced by a central canal which communicates with the fourth ventricle at the calamus scriptorius.

STRUCTURE OF THE NERVE TRUNKS AND GANGLIA

The protoplasmic processes called dendrites have a similar structure to the cell body. The dendrites branch dichotomously, become rapidly smaller, and usually end at no great distance from the cell body.

The axone or axis cylinder process, differs from the cell body and from the dendrites. It does not contain chromophilic granules. It consists entirely of neurofibrils and perifibrillar substance. It emerges from the cell at an enlargement known as the axone hill. This hill is free from chromophilic bodies.

Nerves are divided into two kinds, medullated and non-medullated.

Medullated nerves are divided into two kinds: medullated nerves with a neurolemma, and medullated nerves without a neurolemma.

Medullated nerves with a neurolemma consists of an axone, a medullary sheath, and a neurolemma. A delicate membrane called an axolemma, or periaxial sheath envelops the axone. The medullary sheath called myelin, is semifluid, somewhat resembling fat. The outer covering is the neurolemma, or sheath of Schwann. It is a delicate, structureless membrane which incloses the myelin. There is under this sheath an occasional oval nucleus. At intervals there are constrictions, or nodes, called the constrictions of Ranvier. The part between the nodes is called the internode.

The medullated axones without a neurolemma are the medullated nerve fibers of the central nervous system.

The non-medullated axones, or non-medullated nerve fibers, are divided into non-medullated axones with and those without, a neurolemma.

The non-medullated axone without a neurolemma is merely a naked axone. They are confined to the gray matter and to the beginnings and endings of sheath axones, all the latter being uncovered for a short distance after leaving the nerve cell body and also just before reaching their terminations.

The long axones serve to make connections with the peripheral, or distant, nerve cell, muscle cell, or gland cell; while the shorter axones of certain neurones divide into terminal branches in the immediate vicinity of its cell body, presumably to come into relation with other nerve cells in the same or adjacent groups.

FIG. 77.—Histological structure of tissues.

1. Non-striated or involuntary muscle cell. a. 1, Nucleus; 2, Protoplasm. b. Transverse section showing nuclei in the center of the cells that are cut through the nuclear zone. c. A cross-section of the connective tissue which binds together the muscle cells. d. A section showing the so-called intercellular bridges.

2. A cross-section of the rectus abdominalis. 1, The endomysium which binds the cells into primary fasciculi or bundles. 2, The perimysium which surround the bundles of fasciculi. 3, The epimysium which surrounds the muscle. These binding structures are white fibrous connective tissue.

3. A longitudinal section at the juncture of a muscle and tendon. a, The juncture between the muscle and tendon. Note the many nuclei in both muscle and tendon. This is voluntary or striated muscle which make up the dermal, dermo-osseous and skeletal muscles.

4. A longitudinal section of heart muscle. a, Connective-tissue cell. b, Nucleus of a muscle cell. c, Cement line between the muscle discs. d, The cell or segment.

5. A bipolar ganglionic nerve cell. a, The nucleus. b, The nucleolus. c, The fibrillar structure. d, The medullary sheath.

6. A diagram showing the scheme of the peripheral nerve trunk. a, The neuraxis of the peripheral sensory neurone. b, The spinal ganglion of the superior or sensory root. c, The dendrite or peripheral nerve fiber of the sensory nerve. d, The nerve trunk. f, The sympathetic nerve ganglion connected with the spinal cord through the white and the gray ramus communicans. e, The neuraxis of the sympathetic neurone. g, Neuraxis or trunk of the motor neurone or nerve cell. h, The anterior horn of the gray matter of the spinal cord.

7. A diagram of a peripheral sensory neurone. a, The neuraxis which ends in the spinal cord or brain. b, The T-shaped division of Ranvier. c, The dendrite or sensory nerve fiber in the nerve trunk. d, The nucleus and nucleolus of the cell e., f. the axis cylinder process of the cell. g, The telodendrions or terminal branches of the dendrite or axis cylinder.

8. A schematic diagram of the sensory motor reflex. a, The telodendria. b, The dendrite. c, Nerve cell of the motor neurone. d, The motor neurone. f, The muscle fiber. g, The neuraxis of both sensory and motor neurones, the upper being the sensory. h, The nerve cell in the sensory ganglion. i, The sensory neurone or axis cylinder (nerve fiber). j, The skin with peripheral telodendrion of sensory neurone. ]

Neurones are devoted to the maintenance of functions. Reproductive neurones are so arranged as to receive afferent nerve impulses from other tissues; emissive neurones give off efferent nerve impulses. The former are sensory neurones; the latter are motor neurones if connected with muscles, and excito-glandular if connected with gland cells. The basis, then, of the nerve system is a series of neurones, with projecting and association processes, coördinated for the purpose of performing specific actions manifested either by motion, by trophic changes, or by the apperception of stimuli of a chemic, mechanic (tactile and auditory), thermal, or photic nature.

The whole of the nerve structure is composed of the nerve tissue and supporting connective tissue. The neurones constitute the nerve tissue, while the supportive tissue is composed of neuroglia and of white fibrous tissue derived either from the investing membrane or from the sheaths of its numerous vascular channels.

The neurones, or nerve cells, exhibit marked variations as to external characters, dimensions, and form. The neurone presents a swollen cell mass and a nucleus; it is known as the ganglion cell. From this cell body are given off a number of processes of two distinct kinds: first, protoplasmic processes, which are commonly branched, called dendrites; second, a single, thinner, and paler process; the axis cylinder process.

The bodies of cells vary in size from 4 to 100 microns or more in diameter. The largest cells occur in the inferior horn of the spinal cord, in the spinal ganglia, in the large pyramidal cell layer of the cerebral cortex, in the Purkinjean cell layer of the cerebellum, and in Clark’s column of the spinal cord. Very small cells occur in the olfactory bulbs, in the granular layer of the cerebral and cerebellar cortex, and in the gliosum cornuale of the cord.

Nerve cells are classified according to the number of processes arising from the cell body, and neurones are referred to as unipolar, bipolar, and multipolar.

Unipolar cells are met with frequently in early stages of embryonic development, but are rare in adults, occurring only in the retina, in the olfactory bulb, and within the baskets of the Purkinjean cells of the cerebellum. The cells of the cerebro-spinal ganglia, except the cochlear and vestibular, are apparently unipolar, but they are developmentally and functionally of bipolar nature.

Bipolar cells are found almost exclusively in the peripheral sensory system, as in the olfactory membrane, in the retina, in the cochlear and vestibular ganglia, and in the cerebro-spinal ganglia of the embryo.