DENSITY OF MUSCLE-SPINDLES IN THE JAW MUSCLES OF THE JAPANESE FLYING SQUIRREL AND THE GUINEA-PIG

The number of muscle spindles and their distribution pattern in the ja w muscles were examined in serial sections of the heads of the Japanes e flying squirrel and the guinea pig, and were compared with each othe r. The Japanese flying squirrel had 127 to 177 spindles, the length of which ranged from 140 mum to 1400 mum; 74 to 99 in the masseter, 39 t o 47 in the temporalis, 11 to 28 in the zygomaticomandibularis, zero t o one in the maxillomandibularis and two to three in the medial pteryg oid muscles. The guinea pig had 322 to 346 spindles, the length of whi ch ranged from 140 mum to 2800 mum; 116 to 125 in the masseter, 15 to 16 in the temporalis, 98 to 107 in the zygomaticomandibularis, 89 to 9 4 in the maxillomandibularis, and three to five in the medial pterygoi d muscles, The lateral pterygoid, mylohyoid and anterior digastric mus cles were devoid of spindles, as was the case in the other rodent spec ies examined.