1 504 HUMAN ANATOMY. 



whose fibres are inserted into fibrous tissue completing the lateral wall of the tube 

 (Fig. 1262), this part of the muscle being designated the dilator tub a. In addition 

 to opening the tube, the levator palati elevates its floor. 



The blood-vessels of the Eustachian tube include the arteries, which arise from 

 the ascending pharyngeal and from the middle meningeal and the Vidian branches of 

 the internal maxillary; and the veins, which communicate with those of the tym- 

 panum and of the pharynx and also form a plexus connecting with the cavernous 

 sinus. 



The nerves are supplied from the tympanic plexus and from the pharyngeal 

 branches from the spheno-palatine ganglion. 



THE MASTOID CELLS. 



The antrum tympanicum communicates posteriorly with a variable number of 

 irregular pneumatic cavities, the mastoid cells (cellulae mastoideae), so called because 

 the majority of these spaces occupy the mastoid process. Unlike the antrum, these 

 cells are not developed at birth. As the mastoid process develops, the original 

 diploetic structure is usually more or less replaced by larger cavities forming the 

 pneumatic type. In a study of one thousand bones, Randall found that scarcely two 

 per cent, of mastoids could be classed as diploetic, and only some ten per cent, as 

 combining a notable amount of diploe with pneumatic spaces ; further, that no mastoid 

 is absolutely pneumatic, although some senile bones show a single thin-walled cell 

 occupying the greater part of the process. The pneumatic cells of this region may 

 extend to the sigmoid portion of the lateral sinus ; into the occipital bone ; into the 

 squamous portion of the temporal bone and above the external auditory canal ; into 

 the root of the zygomatic process ; into the floor of the Eustachian tube close to the 

 carotid canal, and occasionally as far as the apex of the petrous portion of temporal 

 bone. These spaces are lined by a very thin mucous membrane, which is continu- 

 ous with that of the antrum and of the tympanic cavity. It is closely united with 

 the periosteum and possesses a layer of low nonciliated squamous epithelium. 



The blood-vessels supplying the mastoid cells are the arteries derived from 

 the stylo-mastoid and the middle meningeal, and the veins, which communicate with 

 those of the tympanum and the external wall of the mastoid process. Some of the 

 veins are tributary to the mastoid emissary and the lateral sinus, whilst others pass 

 beneath the superior simicircular canal through the cranial wall to join the dural veins. 



The nerves are the mastoid ramifications of the tympanic plexus. 



Practical Considerations : The Tympanum. This cavity is continuous 

 anteriorly with the nasopharynx by way of the Eustachian tube, and posteriorly 

 with the mastoid antrum and air cells by way of the attic, so that infection, which 

 is very common in the pharynx, may extend throughout this whole tract. The 

 tympanic cavity extends above the limits of the membrane about 5-6 mm. as the 

 attic, and about 2-3 mm. below as the "cellar" or hypotympanic recess. Secre- 

 tions on the floor, therefore, may not be seen through the membrane. The defective 

 drainage which results from the lower level of the floor of the tympanum, as com- 

 pared with that of the external meatus, is one of the causes of the frequency of 

 chronic otitis media with purulent discharge, even after the early evacuation of 

 the products of inflammation in the acute stage. 



On tin- internal wall tin f,ni,i/ nerve passes in a curve over the vestibule in the 

 anide between the roof and inner wall of the tympanum, then downward in the 

 slightly projecting Fallopian canal with a concave turn above and behind the oval 

 window, continuing its course downward at the junction of the posterior and inner 

 wall to emerge Mow from the skuH at the stylo-mastoid foramen. This canal 

 otttn ' "iiMderal.le re>i-taiice to caries in its immediate neighborhood, although the 

 di^easr not infrequently communicates itself to the nerve. Such involvement of 

 the nerve is often tin- prodromal symptom of a fatal cerebral affection (Politzer). 

 At birth this portion <>f the Fallopian canal is very thin and translucent, and is 

 deficient as it an h<> over the oval window, so that involvement of the nerve is 

 much more common in children than in adults. 



