Fig. 8. VIEW OF THE MIDDLE EAR, NASAL CAVITIES AND OTHER DEEP STRUCTURES OF THE HEAD
In this plane that is closer to the midline one ofthe cavities of the head, the maxillary sinus, has disappeared. Instead, the complicated nasal cavity,and other cavities associated with it -the ethmoidal labyrinth (77) and frontal sinus (78)- have appeared in full view. The air cells of the mastoid process (9), the middle ear, and the pharyngo-tympanic tube of Eustachius (87) are exposed. In the middle ear, two of the three auditory ossicles (84) are seen, the handle of the hammer attached to the center of the ear-drum (81) being conspicuous. The lower segment of the drum is cut off and is found in Fig. 9. The Eustachian tube, which is cut lengthwise, is osseous at its tympanic end and is cartilaginous and membranous at its pharyngeal end. At the point where these two sections merge, the passage of the tube is most narrow: the isthmus (87d). Along the osseous part of the tube is the minute tensor tympani muscle (89) that regulates the tension of the eardrum.