192 THE ANATOMY OF THE HORSE 



maxillary division of the 5th nerve. The pouch is lined by mucous 

 membrane continuous with that of the Eustachian tube, and by that 

 tube it communicates with the pharynx and the middle ear. Normally 

 the pouch contains air, which it receives from the pharynx through the 

 Eustachian tube. When the mucqus lining of the pouch becomes 

 ' inflamed pus tends to accumulate in the cavity, since the Eustachian 

 •orifice, by which the inflammatory products might escape into the 

 pharynx, is, in the ordinary position of the head, situated towards the 

 upper part of the pouch. 



The Eustachian Tube (Plate 32). This is a fibro-cartilaginous tube 

 of three or four inches in length, extending downwards from the petrous 

 temporal bone to the pharynx. At its upper extremity the tube com- 

 municates with the cavity of the middle ear (in the temporal bone), and 

 at its lower extremity it opens into the pharynx by a slit-like aper- 

 ture. For nearly the whole of its extent the tube is slit open along 

 its outer side, and is thus in free communication with the guttural 

 pouch. The tube is lined by mucous membrane, and through its agency 

 air is admitted from the pharynx to the guttural pouch and the middle 

 ear. 



The Internal Maxillary Artery (Plates 31 and 32) results from the 

 division of the external carotid on the outer surface of the great cornu 

 of the hyoid bone. It is much larger than the superficial temporal, 

 which is the other terminal branch of the external carotid. In passing 

 to enter the subsphenoidal canal, it describes a double or sigmoid curve, 

 — the first convex downwards, the second upwards. In this course the 

 artery is placed within the articulation of the jaw and the external 

 pterygoid muscle, and rests successively on the guttural pouch and the 

 tensor palati muscle. It is crossed superficially by the inferior dental 

 and lingual nerves, and deeply by the chorda tympani. It detaches the 

 following collateral branches : — 



1. The Inferior Dental Artery, a large branch arising from the convexity 

 of the first curve. It has already been seen entering the inferior dental 

 canal. 



2. The Tympanic Artery, the smallest of the branches here given off, 

 is detached at nearly the same point as the preceding, but from the 

 opposite side of the parent vessel. It lies beside the chorda tympani 

 nerve, and penetrates the petrous temporal bone to be distributed in the 

 tympanum or middle* ear. 



3. The Great Meningeal (spheno- spinous) Artery, a vessel of variable 

 volume detached from the upper side of the parent artery beneath the 

 inferior maxillary nerve, and entering the cranial cavity by the foramen 

 lacerum basis cranii. 



4. The Pterygoid Arteries, two or three, arising from the concavity of 

 the second curve. 



