Components of the mylohyoid nerve degenerate during evolutionary progress in primates

The vibrissal, cutaneous and muscular branch of the mylohyoid nerve was studied on both sides in 67 individuals in 16 species of 14 genera (Rattus, Canis, Felis, Suncus, Talpa, Tupaia, Lemur, Nicticebus, Galago, Pithecia, Ateles, Papio, Macaca, Pan) with particular reference to the phylogenic change of these branches. In this series of studies in rats, dogs, cats, tree shrews, and 5 sides out of 30 crab-eating monkeys, a fine branch of the mylohyoid nerve proceeds to the vibrissal hair follicle. The vibrissal branch passes through the platysma and enters the vibrissal hair follicle at the middle of the body. It is conceivable that the intermandibular vibrissae had been reduced from the intermandibular skin during the phylogenic progress of prosimians. The vibrissal branch of the mylohyoid nerve may retire from the intermandibular region according to the disappearance of the intermandibular vibrissae. The cutaneous branches of the mylohyoid nerve were detectable in almost all of the specimens. The cutaneous branch seems to be retained despite the reduction of the intermandibular vibrissae in primates. It must be considered to retire from the intermandibular skin during the evolutional progress of the hominids. In spite of the alteration of the muscular arrangement, the mylohyoid nerve is preserved as the innervation to the anterior belly of the digastric muscle and to the mylohyoid muscle in all specimens examined.