Abstract
Though the literature contains many accounts of the order in which tracts of the central nervous system acquire their medullary sheaths, Anderson's 1 study of the medullation of the sympathetic system appears to be the only record of observations on the order in which peripheral nerves acquire their sheaths. Stimulated by these observations, a study has been made of the sequence in which systems of fibres in the somatic as well as in the visceral nerves become myelinated in the kitten.
The peripheral nerves of fetal kittens ranging between 8 cm. snout-rump length and full term (c.a. 14 cm.) have been examined either by teasing them out in glycerine after staining for 12 hours in 1% osmic acid, or by sectioning in paraffin, following treatment by the Weigert technic.
The following summary presents the more significant findings in the specimens examined, grouped according to their lengths:
Fetuses 8.0 cm. No medullation was found in any of the peripheral nerves.
Fetuses 9.5 cm. (a) Phrenic nerve contained about 35 faintly medullated fibres. (b) Vagus contained about 12 medullated fibres in the middle of the neck. (c) A few medullated fibres were found in the spinal portion of the eleventh. (d) Medullated fibres were present in all the cervical and dorsal anterior roots. (e) Posterior roots of the cervical segments were medullated only near the ganglion. No medullation was observed on the more caudal dorsal roots.
Fetuses 10.5 cm. (a) The third, fourth, sixth and twelfth nerves contain a few medullated fibres. (b) Medullated fibres were found in both the motor and sensory roots of the trigeminal. (c) None of the fibres of the posterior roots are medullated to their entrance into the cord.
Fetuses 11.5 cm. (a) The phrenic nerve was medullated close to the diaphragm.
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