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Histological examination of the jaw muscles of the Japanese ermine showed that 4 jaw-closing muscles have 13 muscle spindles on one side of the face. The temporal muscle has 99 muscle spindles, 68 being in the anterior vertical and 31 in the posterior horizontal belly. The masseter muscle has 33 muscle spindles, 23 being in the profound and 10 in the superficial belly. The medial pterygoid muscle has 7 muscle spindles and the zygomaticomandibular muscle contains 4 muscle spindles. The lateral pterygoid and the jaw-opening muscles have no spindles.

The axonal transport method of the horseradish peroxidase (HRP) was used for defining the anatomical background of the snout movements in 54 Japanese shrew-moles, Urotrichus talpoides, 4 Temminck's moles, Mogera wogura, and 21 big-clawed shrews, Sorex unguiculatus. Either snout muscle or proximal stump of the facial nerve on one side was exposed to HRP. The results obtained from this experimental study were as follows: HRP-containing cells were found either ipsilaterally or bilaterally in the facial motor nucleus in both the moles and shrews. The HRP-containing cells were grouped into 4 subnuclei: ventrolateral, ventromedial, dorsal and accessory. In case of the snout muscle being exposed to the HRP were the HRP-containing cells found in the ventral groups. A HRP-positive fiber bundle was found at the genu of the facial motor root extending toward the median raphe, when the proximal stump of the facial nerve was exposed to the HRP.

In the masticatory muscles, neuromuscular spindles have a very important role in controlling the jaw movement since they act as stretch receptors in skeletal muscles. The continuous intake of fine-grained diet which is easily chewable leads to degeneration of the sensory endings of Ia fibers in many muscle spindles of the mouse masseter muscle in only 120 d after birth.