Muscles involved in naris dilation and nose motion in rat.

In a number of mammals muscle dilator nasi (naris) has been described as a muscle that reduces nasal airflow resistance by dilating the nostrils. Here we show that in rats the tendon of this muscle inserts into the aponeurosis above the nasal cartilage. Electrical stimulation of this muscle raises the nose and deflects it laterally towards the side of stimulation, but does not change the size of the nares. In alert head-restrained rats, electromyographic recordings of muscle dilator nasi reveal that it is active during nose motion rather than nares dilation. Together these results suggest an alternative role for the muscle dilator nasi in directing the nares for active odor sampling rather than dilating the nares. We suggest that dilation of the nares results from contraction of muscles of the maxillary division of muscle nasolabialis profundus. This muscle group attaches to the outer wall of the nasal cartilage and to the plate of the mystacial pad. Contraction of these muscles exerts a dual action: it pulls the lateral nasal cartilage outward, thus dilating the naris, and drags the plate of the mystacial pad rostrally to produce a slight retraction of the vibrissae. On the basis of these results, we propose that muscle dilator nasi of the rat should be re-named muscle deflector nasi, and that the maxillary parts of muscle nasolabialis profundus should be referred to as muscle dilator nasi.