The axones of the cochlear and vestibular nerves which rise in the spiral and vestibular ganglia are medullated fibers. They carry impulses of hearing and equilibrium to their terminal nuclei in the medulla oblongata.
The dendrites of fusiform bipolar neurones are single and cellipetal in conduction. Those' from the olfactory ganglion are very short (0.02 mm.). In the form of a tuft of hairs, they protrude slightly through the mucous membrane into the nasal fossa. The dendrites of the retinal bipolars measure from 0.01-0.015 mm. Branching they form contacts with the rods and cones, from which they receive the visual impulses. The dendrites of the spiral and vestibular neurones are longer, measuring a few millimeters in length. They are in part medul lated. They extend from the spiral ganglion to the hair cells of the spiral organ of Corti and from the vestibular ganglion to the hair cells of the acustic spots and ampullary crests within the labyrinth. From the hair cells they receive the impulses of hearing and equilibrium.
Pyriform or Bipolar pear shaped bipolars are the neurones of the common sensory and taste nerves: their cell-bodies form the ganglia of those nerves; their processes constitute the fibers of them. Formerly these bipolars were called unipolar neurones with two axones, in accordance with their mature appearances; but their embry ology and their phylogeny show them to be true bipolars, and the evolution of the peripheral process and its cellipetal conduc tion furnish abundant reason for calling it a "dendrite" rather than an "axone with reversed polarity (Figs. 65A, 66 and 67)." The cell-body of the pear-shaped bipolar neurone is very large (170 ,c). It is the product of an evolution which con tinues after the simple fusiform stage is reached: the spindle shape is the permanent form in the cyclostome and amphyoxus (J. B. Johnston), as it is the embryonic form in man. In the bony fishes some of the peripheral sensory neurones become, pear-shaped; in man practically all take this form. The cell body becomes almost spherical in this process; the two processes shift to one side of the body; and, by the growth of the axone and dendrite, the body is pushed in the direction of least re sistance, the two processes are approximated and, therefore, appear connected with the body by a common stem. The
common stem joins the cell-body at a highly developed axone hillock and, after a sinuous course of z or 2 mm., it bifurcates into its two original parts, axone and dendrite. The nuclear and cytoplasmic constituents of the pear-shaped bipolar neu rones are identical in kind with those in multipolars.
The axones of pear-shaped bipolar neurones form the roots of the common sensory and taste nerves. They extend to the surface of the cerebrospinal axis covered by a neurolemma and a segmented myelin sheath, if medullated. There the neuro lemma ends, but the fibers enter the axis with solid medullary sheaths. Inside the axis the axones divide into ascending and descending T-branches, which give off collaterals; after a course of varying length the myelin sheath is lost, the telodendria are formed and synapses are established with neurones of the brain or cord to which the afferent impulses are delivered. Cellifugal conduction is again exemplified in the axone.
The dendrite of a pear-shaped bipolar neurone is very long and slender. It extends from the common stem of a cell-body in a spinal or cranial ganglion out to some part of the periphery.
It is an afferent or sensory fiber in some cranial or spinal nerve. In every microscopic particular it is like the axone of a first type multipolar neurone (q.v.). Its telodendria are free among the cells of the various tissues or they are encapsulated by special ized end-organs, viz., the tactile, bulbous and lamellous cor puscles, and the neuro-muscular and neuro-tendinous spindles. The endings are adapted to the reception of external and in ternal stimuli; the impulses thus excited travel up the dendrite to the cell-body and continue through the axone into the cerebrospinal axis. Like all dendrites, these possess cellipetal conduction.
The neurone doctrine maintains that every neurone is de rived from an epiblastic cell; that nerve fibers are outgrowths of the cell-body; and that the individual neurones in all higher animals are related to each other only by contact (see "Neu rones and the Neurone Concept"—Santee: Illinois Medical Journal, June, 1912).