70 TOPOGRAPHICAL ANATOMY OF THE 



into a gap between the pharyngeal muscles, and is, consequently, in 

 direct contact with the diverticulum of the auditory tube. The ventral 

 part (pars digestoria) is both respiratory and alimentary in function, 

 inasmuch as it communicates with the mouth and dorsal part in the 

 one direction and with the cesophagus and larynx in the other. On 

 each side of the entrance to the larynx there is a narrow but deep 

 piriforyn recess (recessus piriformis). The mucous membrane of the 

 upper part of the cavity is redder than that of the more ventral and 

 caudal part. 



There are seven openings into the cavity of the pharynx. Of these 

 the opening from the mouth (isthmus faucium), the opening into the 

 larynx (aditus laryngis) and the continuation of the cavity into the 

 cesophagus, are single and median, and occur in the pars digestoria. 

 Two paired openings, namely, from the nasal cavities (choanse) and from 

 the auditory tubes, are present in the dorsal part. 



The openings of the auditory (Eustachian) tubes, the ostia^ 

 pharyngea tuhce, are slit-like apertures, covered medially by the 

 flattened terminations of the cartilaginous wall of the tubes. A small 

 oblique fold of mucous membrane (plica salpingopharyngea) runs down- 

 wards from each opening towards the larynx. 



Dissection. — The small opening that was previously made in the wall 

 of the diverticulum of the auditory tube should be extended by a hori- 

 zontal incision sufficiently large to show the whole of the interior of the 

 cavity. Make the incision half-way between the stylo-pharyngeal muscle 

 and the auditory tube. 



The auditory (eustachian) ^ tube and its diverticulum — 

 Diverticulum tithre auditivfu. — The diverticulum of the auditory tube 

 is a thin-walled sac, capable of holding some 300 cc. of fluid, formed by 

 the extrusion of the mucous membrane of the tube out of an elongated 

 slit in its ventral wall. Its occurrence, among domesticated mammals, 

 is peculiar to the Equidce. 



The diverticulum occupies the whole of the space between the base 

 of the cranium, the atlas and the pharynx ; and (except where the 

 ventral straight muscles of the head intervene) it comes into contact 

 with its fellow in the median plane, where a thin partition, formed by 

 the fusion of the mucous linings of the two diverticula, is all that separ- 

 ates the interior of the two cavities. 



The anterior limit of the cavity is in the form of a small blind pouch 

 immediately ventral to the body of the anterior part of the sphenoid 

 bone and a short distance from the choanse ; or, expressed in terms of 



1 Ostium (dim. of os, a mouth) [L.], a .small opening. 



2 Bartolomeo Eustachio, an Italian anatomist, 1520-1574. 



