The axone, neuraxone or of a multipolar neurone is the first process to develop; it attains considerable length before there is any evidence of dendrites. It grows out of what was primarily the deep end of the columnar cell, as its function in the simplest forms is to convey to muscles or interior organs impulses set up by environmental stimuli. If the axone is long, the neurone is of the first type (type of Deiters); if the axone is short and breaks up at once into many branches, it constitutes a second type neurone (type of Golgi). The axone of a Type II neurone branches dichotomously at acute angles near the cell-body, arborizing like a tree; it is non-medullated; and it terminates in many pointed branches in contact with other neurones, to which it carries its impulses. Some of the second type neurones in the cerebral cortex have a brush-like axone at one end and similar dendrites at the other; those are the double-brush cells of Cajal. The axone of a Type I neurone is relatively long, from r—roo cm. It is slender, smooth and uniform in size; it is fibrillar in character; it enters into a fas ciculus within the cerebrospinal axis and into a nerve outside the axis; it gives off collaterals at right angles to its course, which like the parent axone end in the form of a brush or tassel, called end-brush or telodendrion; the end-brushes form contacts (synapses) with another neurone, a muscle cell or a gland cell, and deliver to them their impulses. The telodendria present various modifications: they enter into motorial end-plate on striated voluntary muscle; they form "climbing fibers" which entwine about the adjacent neurone, and form pericellular baskets in other situations, etc. All axones are cellifugal in their conduction (cella—a cell; and fugere—to flee from). The axone is composed chiefly of neurofibrillx and mitochondria imbedded in neuroplasm and surrounded by a delicate fibrous sheath, the axilemma. Very many axones become medullated in the cranio-spinal system; they are non-medullated in the sympathetic system. The myelin sheath begins a short dis tance from the cell-body and terminates proximal to the telo dendria. It is a solid, continuous sheath inside the brain and spinal cord and in the optic nerve and tracts. The medullated nerves which possess the primitive sheath of Schwann, called the neurolemma, have segmented myelin sheaths; the segments measure from o.o8-1 mm. in length. The subdivisions of these segments by the Schmidt-Lantermann lines or cones are probably artefacts. A neurolemma invests the axones of all peripheral nerves except the optic and olfactory; the former is medullated, the latter is not, but bundles of its axones are sur rounded by a sheath like a neurolemma. No neurolemma is found within the brain and spinal cord. The constrictions of the neurolemma which separate the segments of myelin are the nodes of Ranvier. At the nodes the neurolemma immediately invests the axilemma, and collaterals are given off at these points.
The dendrites of a multipolar neurone always appear later than the axone of the same cell. The pitcher-shaped cells of Purkinje in the cerebellum have but two richly branched dendrites; other multipolar neurones have many dendrites.
These dendrites taper rapidly and branch freely at acute angles, like a tree, hence are named dendrites (dendron—a tree). They are partial protrusions of cytoplasm, which arborize close to their origin and possess the neurofibrillm and tigroid substance of the cell-body. In contour they are irregular, being gemu lated and varicose. They terminate in fine beaded points in
contact with the end arborizations of other neurones from which they receive impulses. All dendrites conduct impulses toward the cell-body, all possess cellipetal conduction (cella a cell, and petere—to seek). The dendrites of multipolar neurones are destitute of both the neurolemma and the medul lary sheath.
II. Bipolar neurones (Figs. 65 and 69) are the peripheral neurones of the sense organs; they form the special and common sensory nerves. Bipolar neurones originate in the neural crest and in homologous portions of epiblast (see note).
NOTE.—The dorso-lateral plaques (or placodes) of epiblast which form the vestibular and cochlear ganglia in man and represent the anlagen of the nervus lateralis in gill-breathing animals: the epibranchial plaques that form the ganglia of the nerves of taste (the glosso-palatine or intermediate and the glosso-pharyngeal): and the epiblastic plaques in the olfactory pits which constitute the ganglia of the olfactory nerves are all derived from out-lying parts of the neural plate—parts homologous with the neural crest. And the retina, which in man is a diverticulum of the prosencephalon, is derived from epiblast corresponding to neural crest; this part of the neural crest is included in the formation of the cephalic end of the neural tube because of its great size in man. So the bipolar neurones of the olfactory, optic, gustatory and acustic nerves, which in man appear not to originate in the neural crest, rise from parts homologous with it.
The most primitive and embryonic bipolars are those whose cell-bodies form the acustic and olfactory ganglia and the in ternal nuclear layer of the retina (the layer of bipolars). They have fusiform or spindle-shaped cell-bodies. The specialized bipolar neurones have pear-shaped bodies which form the gan glia of the common sensory and taste nerves. The bodies of the bipolars have the same cellular constituents as the multi polar neurones; and the life histories of the two neurones are identical both as to the points of origin of the axone and the dendrite and as to sequence in the time of their development. The bodies of bipolar neurones are inclosed in a nucleated cap sule continuous with the neurolemma of the processes. The neurolemma invests the neurone from the surface of the cere brospinal axis to the vicinity of the telodendria; it bounds a perineural lymph space.
The fusiform bipolar neurones are prolonged at opposite poles into two processes, the axone and dendrite. The axone grows out of the deep end of the neuroblast into the brain. It divides into T-branches which give off several collaterals ending in telodendria; the telodendria form synapses with the neurones of a terminal nucleus, to which the impulses travers ing the axone are delivered (cellifugal conduction).
In the olfactory nerve the axones are very slender (0.5 They are varicose, non-medullated and are collected into twenty or more bundles that are invested by a nucleated sheath like a neurolemma. They end in the olfactory bulb.
The optic nerve morphologically is really formed by the bipolars of the retina, whose axones are very short; they end in contact with the ganglionar neurones, whence the optic nerve, as it is ordinarily described, takes its origin. The fibers of the optic nerve and tracts correspond to a brain tract, so they have no neurolemma; but they are large and medullated and are imbedded in neuroglia. They terminate in the inter brain and mid-brain.