AUDITOKY TUBE. 



837 



from the transverse sinus by a transparent lamella which, in some instances, is 

 partly deficient. In other eases the cells may be small and numerous, invading 

 only a portion of the process, the remainder consisting of diploetic tissue ; 

 occasionally a solid mastoid is met with. No definite conclusion can be formed 

 as to their condition by external percussion or examination. The air-cells are 

 not limited to the mastoid portion of the temporal bone, but extend forwards over 

 the roof of the meatus, upwards towards the squama temporalis, and medially towards 

 the temporo-occipital suture ; occasionally they invade the pars jugularis of the 

 occipital bone. The tympanic antrum and the mastoid air-cells are lined with 

 thin mucous membrane, continuous with that of the tympanic cavity ; the deep 

 surface of the mucous membrane is fixed to the periosteum ; its free surface is 

 covered with a layer of flattened, non-ciliated epithelium. 



TUBA AUDITIVA. 



The auditory tube (O.T. Eustachlan tube) leads from the tympanic cavity to 

 the nasal part of the pharynx, and transmits air to the former, in order that the 

 pressures on the medial and lateral surfaces of the membrana tympani may be 

 equalised ; it may also serve to convey mucous secretion from the tympanic cavity. 

 Its tympanic orifice (Fig. 709) opens into the anterior part of the tympanic cavity, 

 below the semicanal for the tensor tympani muscle. Directed downwards and medial- 

 wards, the tube ends on the upper part of the nasal part of the pharynx by a wide 

 pharyngeal orifice (Fig. 674, p. 803). It measures about 35 mm. in length, and forms 

 with the horizontal plane an angle of 30 to 40, with the sagittal plane an angle 

 of about 45, and with the bony part of the external acoustic meatus an angle of 135 

 to 140. It consists of two portions: (a) an antero-medial, the pars cartilaginea 

 tubae auditivae, having a length of about 25 mm. ; and (b) a postero-lateral, the 

 pars ossea tubae auditivae, measuring about 10 mm. in length. The two portions are 

 not in the same plane, the cartilaginous part inclining downwards a little more 

 than the osseous portion, and forming with it a wide angle. The lumen of the tube 

 is widest at the pharyngeal orifice, narrowest 

 at the junction of the bony and cartilaginous 

 portions, forming here the isthmus, and 

 again expanding towards the tympanic 

 cavity ; hence it presents, on longitudinal 

 section, somewhat the appearance of an 

 hour-glass. The pars ossea occupies the 

 angle between the tympanic part and the 

 petrous part of the temporal bone, and is 

 separated by the septum canalis musculo- 

 tubarii from the semicanal containing the 

 tensor tympani muscle ; medial to it is the 

 carotid canal. The pars cartilaginea con- 

 sists partly of cartilage and partly of 

 fibrous membrane. The cartilage of the 

 auditory tube is an elongated triangular 

 plate, of which the apex is firmly attached 

 to the medial end of the pars ossea, while 

 the base is free, and forms a projection on 

 the upper and posterior aspects of the 

 pharyngeal orifice. The upper edge of this 

 cartilaginous plate is bent laterally in the 

 form of a hook, and so produces a furrow 

 open below, the furrow being converted into a complete canal by the membranous 



.na of the tube. On transverse section (Fig. 713) the cartilage presents two 

 .aminae continuous with each other superiorly : (a) lamina medialis, broad and 



ik ; and (&) lamina lateralis, thin and hook-shaped. At the pharyngeal orifice 



lamina medialis forms the entire medial wall of the tube, but it gradually 

 liminishes in breadth on approaching the isthmus tubae. Fissures are often seen 



54 & 



Mucous 

 glands 



FIG. 713. TRANSVERSE SECTION OF THE 

 CARTILAGINOUS PART OF THE AUDITORY TUBE. 



