Nerve fiber caliber analyses in the mouse ramus digastricus and rami stylohyoidei with the electron microscope

2 nerve branches to the m. stylohyoideus are found in the mouse. One of them (r. stylohyoideus proximalis) arises from the posterior side of the truncus facialis just under the crossing and contacting region with the r. auricularis n. vagi and just proximal to the origin of the r. digastricus. It passes postero-medially along the r. auricularis n. vagi and enters the region of the origin of the m. stylohyoideus. The other branch (r. stylohyoideus distalis) arises as a common branch with the r. digastricus, passes postero-media-inferiorly and separates from the r. digastricus on the way. The myelinated fibers of the r. digastricus and the rr. stylohyoidei derive from the large fiber zone of the truncus facialis. But they are considerable larger than those in the large fiber zone. While they are scattered in the more proximal truncus facialis, they gradually become crowded as they come near to the origins of these rami. On the average, the r. stylohyoid, prox., the r. stylohyoid. dist., and the r. digast. contain 15, 16 and 49 myelinated fibers and 5, 13 and 24 unmyelinated fibers respectively. About 67% of the myelin sheaths of the r. digast. and about 71% of those of the rr. stylohyoid. prox. et dist. are 3-5.5 micron in minor diameters. About 64% of the axons of the myelinated fibers of these rami are 2-4 micron in minor diameters. The frequency distributions of minor diameters of the myelin sheaths and axons of the myelinated fibers of these rami show that they are approximately 1 micron larger than those of the large fiber zone of the motor root of the n. facialis and the truncus facialis distal to the foramen stylomastoideum.