756 THE SENSE ORGANS AND SKIN OF THE HORSE 



below the level of the posterior nares. It is a slit, an inch or more (ca. 3 cm.) 

 in length, which slopes downward and backward. It is bounded internally by the 

 thin free edge of the tube, from the lower part of which a fold of mucous membrane 

 (Plica salpingo-pharyngea) extends in the same direction on the lateral wall of 

 the pharynx for a distance of a little more than an inch usually. The outer boun- 

 dary of the opening is the lateral wall of the pharynx. The basis of the tube is a 

 plate of fibro-cartilage (Cartilago tului^ auditivse) which is firmly attached dorsally 

 to the fibrous tissue Avhich closes the foramen lacerum basis cranii, the temporal 

 wing of the sphenoid, and the pterygoid bone.^ On cross-section the cartilage 

 (except at its tympanic end) is seen to consist of two laminae which are continuous 

 with each other above. The internal lamina (Lamina medialis) gradually widens 

 toward the })haryngeal end, where it forms a broad valvular flap. This is convex 

 internall}^, and its thin anterior edge forms the inner margin of the pharyngeal 

 opening. Behind this the lamina has a thick free edge which projects ventrally 

 from the roof of the guttural pouch. The external lamina (Lamina lateralis) is 

 narrow and thin, and is related externally to the levator and tensor palati muscles, 

 which are in part attached to it; it does not exist at the pharyngeal end of the tube. 

 The mucous membrane of the tube is continuous behind with that of the tympanum 

 and in front with that of the pharynx. On either side it is reflected to form a large 

 diverticulum, the guttural pouch. It is covered with ciliated epithelium, and con- 

 tains mucous glands and lymph nodules. 



The pharyngeal opening appears to be closed ordinarily, but the tube opens during degluti- 

 tion. This action is apparently produced by the part of the palato-pharyngeus muscle which 

 is attached to the tube. 



The Guttural Pouches 

 The guttural pouches, right and left (Figs. 255, 256, 569), are large mucous 

 sacs, each of which is a ventral diverticulum of the Eustachian tube (Diverticulum 

 tuba3 auditivse) ; they are not present in the domesticated animals other than the 

 eciuidffi. They are situated between the base of the cranium and the atlas above 

 and the pharynx below. Medially they are in apposition in great part, but are to 

 some extent separated by the intervening ventral straight muscles of the head. 

 The anterior end is a small cul-de-sac which lies below the body of the presphenoid 

 between the Eustachian tube and the median recess of the pharynx. The posterior 

 extremity lies near or below the atlantal attachment of the longus colli. The pouch 

 is related dorsally to the base of the cranium, the atlanto-occipital joint capsule, 

 and the ventral straight muscles. Ventrally it lies on the pharynx and the origin 

 of the oesophagus. Laterally the relations are numerous and complex. They 

 comprise the pterygoid, levator palati, tensor palati, stylo-hyoideus, and digastricus 

 muscles; the parotid and submaxillary glands; the external carotid, internal 

 maxillary, and external maxillary arteries; the internal maxillary and jugular veins; 

 the pharyngeal or guttural lymph glands; the glosso-pharyngeal, hypoglossal, and 

 superior laryngeal nerves. The vagus, accessory, and sympathetic nerves, the 

 superior cervical ganglion, the internal carotid artery, and the inferior cerebral vein 

 are situated in a fold of the ui)per wall of the pouch. The pouch is reflected around 

 the anterior border of the great cornu of the hyoid bone so as to clothe both surfaces 

 of the upper part of the latter. It thus forms an outer compartment, which extends 

 backward external to the great cornu and the occipito-hyoideus muscle; it is 

 related externally to th(> parotid gland, the articulation of the jaw, the ascending 

 part of the internal maxillary artery, the superficial temporal artery, and the facial 

 nerve; dorsally it covers the mandibular nerve and its chief branches, and is at- 

 tached to the styloid process of the conchal cartilage. 



' There is, strictly speaking, no osseous part of the tube such as occurs in man. In the horse 

 the cartilage extends to the tympanic orifice. 



