The myelin sheath of axones and dendrites, which is devel oped as the neurone begins to functionate, is imbedded in neuroglia within the optic and acustic nerves, and in the brain and spinal cord; but, elsewhere, is surrounded by the neurolemma (Schwann) and the fibrous sheath of Henle. The fibers of the olfactory nerve and most sympathetic fibers are not medullated, but the latter possess the fibrous sheath. Near the cell-body and near the end-tuft the processes are naked, having neither the fibrous nor the medullary sheath. "Myelin is a mixture of complex fats and lipoid substances, some of which are combined with sugar" (F. T. Lewis). It is an emulsion supported by a delicate reticulum of neurokeratin. In preserved specimens it shrinks greatly and fissures. Ether and alcohol dissolve the fats but not the reticulum, which is revealed by such treatment. Osmic acid stains myelin very black (Figs. 68, 69 and 70).
Types of The first type has a long axone, which preserves its identity, though it may give off many col laterals. Found in fasciculi of brain and spinal cord and in nerves (Deiters) (Figs. 65 and 68).
2. The second type has a short axone, breaking at once'into branches of apparently equal importance, the dendraxone. Found in cerebrum and cerebellum (Golgi).
There are probably no neurones that have more than one axone. The double-brush cells of Cajal are really second type cells.
Orders of Neurones.—i. The first order has distal process in relation with the periphery, as spinal-ganglion and anterior col umna neurones, and conducts from the periphery or to it.
2. The second order has cell-body or distal process in relation with neurone of first order. It conducts to a neurone of the first order or conducts centrally from it. In like manner there are neurones of the third, fourth, fifth order, etc.
Functions of Neurones.—i. Afferent. 2. Associative. 3. Efferent. In afferent conduction paths the dendritic side of each neurone is directed toward the periphery to receive the in coming impulses: the axones are directed toward the periphery in efferent paths in order to carry the impulses directly to striated muscle, or to a ganglion, through which it reaches smooth muscle (also heart muscle) and gland cells. Two such paths may be connected by associative neurones and a reflex arc established (Fig. 72).
Degeneration.—Augustus V. Waller discovered in 185o that a nerve fiber, severed from the cell-body out of which it grew, soon undergoes degeneration. This degeneration of Waller is evident in about 48 hours and is almost complete by the four teenth day. It consists of a disintegration of the myelin into droplets and globules of granular lipoid substances which stain very deeply with Marchi's fluid; of a breaking up and gradual disappearance of the nerve fiber; and, later, of an absorption of the myelin debris. In peripheral nerves having a neurolemma,
there is also a proliferation of the nuclei of the neurolemma and the formation of a "band fiber," which guides the "cone of growth" in regeneration. That part of the fiber connected with the cell-body, the central stump, does not suffer this Wallerian degeneration. In peripheral nerves the central stump may very soon show evidence of regeneration; but within the brain and spinal cord regeneration does not take place with any degree of perfection in man. On the contrary, signs of degeneration slowly appear in the cell-body after 10 days and grow more evident to the end of the third or fourth week. This degenera tion consists in a shifting of the nucleus to an eccentric position and a breaking up and disappearance of the Nissl substance, a chromatolysis. It is called Nissl degeneration. In the cerebro spinal axis this Nissl degeneration is usually followed by grad ual atrophy of the whole neurone and, after many months or a period of years, by entire disappearance of it. Nissl degenera tion, followed by gradual atrophy and disappearance, also occurs when neurones are deprived of their function by any cause, as the removal of a limb or organ or the destruction of any group of neurones in a chain.
Regeneration.—If a neurone is destroyed in man it is not replaced by proliferation of other neurones; mature neurones do not exhibit mitotic or direct division. However, complete regeneration may follow the cutting of peripheral nerves. The most interesting proof of this is furnished by the experi ment of Henry Head, in which the superficial radial nerve of his left arm was cut at the elbow and the recovery carefully observed through 567 days (Head and Rivers: Brain, Vol. 31, 1908). The regeneration of a nerve fiber (axone or dendrite) is similar to its original development. A soft "cone of growth" forms on the distal end of the central stump, the part connected with the cell-body; this growing cone by amceboid movement, that is, by sending out and withdrawing one psuedopod after an other, gradually insinuates itself between the cells of the "band fiber" until it reaches every point touched by the original fiber. It appears to be directed by a strong neurotropic force residing chiefly in the cells of the "band fiber." Sustentacular Tissue (Fig. 73).—In the brain and spinal cord and in the optic nerves three forms of sustentacular tissue are found supporting the neurones, viz. : ependyma, neuroglia and ordinary connective tissue. The first two are derived from epiblast, the last is of mesoblastic origin.