THE MIDDLE EAR OR TYMPANUM. 845 



" Two in number, one being on each side, the guttural pouches lie 

 against each other in the median plane, and descend to the larynx, where 

 they terminate in a cul-de-sac constituting their fundus. Before and behind, 

 they extend from the anterior part of the pharynx to the inferior face of the 

 atlas. The capacity of each is about fths of a pint ; but in consequence of 

 the extensibility of the mucous membrane, the extent and capacity of the 

 guttural pouches are particularly variable. 



' Irregular in shape, like the space which it occupies, the guttural pouch 

 corresponds, behind and above, to the base of the occipital and sphenoid 

 bones. When this reservoir is distended, its lower part, or fundus, descends 

 on the lateral portions of the pharynx and larynx, to the lower extremity of 

 the parotid gland, in the loose cellular tissue of that region. 



" Externally, the guttural pouch contracts numerous different relations 

 in the intermaxillary and parotideal regions, and in its posterior portion. 



" a. In the intermaxillary region, it is in relation with the tensor palati, 

 pterygoideus and hyo-pharyngeus muscles, as well as with the internal 

 maxillary artery and lingual nerve; it envelops the large branch of the 

 hyoid bone, and covers the inner face of the internal pterygoideus muscle. 



" 6. In the parotideal region, the guttural pouch responds, above, to the 

 inner face of the parotid gland, from which it is separated by the auricular 

 vessels and nerves ; a little lower, at the posterior angle of the hyoid bone, 

 to the stylo-hyoideus muscle and the styloid process of the occipital bone ; 

 here the* auricular artery passes obliquely upwards and backwards, and 

 the membrane of the pouch is more closely united to the parts covering it. 



" Below this, the guttural pouch is in relation with the stylo-maxillaris 

 muscle, external carotid, and the nerves forming the guttural plexus, such as 

 the ninth and twelfth pairs, the great sympathetic, etc. Lower, it is related 

 to the parotid gland, to the inferior extremity of which it may be prolonged. 



" c. Posteriorly. The guttural pouch is in relation with the atlas, flexor 

 muscles of the head, occipital artery, etc. ; it forms a fold that envelops 

 principally the pneumogastric and sympathetic nerves, and, anteriorly, 

 another fold that incloses the internal carotid. 



" The mucous membrane of the guttural pouches is thicker and stronger 

 than that lining the Eustachian tube and the cavity of the tympanum. 

 Only slightly adherent to the adjacent parts, except at the branch of the 

 hyoid, the inner face of the stylo-hyoideus, etc., it is smooth internally, and 

 lubrified by the mucus it secretes. It may become the seat of purulent 

 collections, which compress the larynx and obstruct the respiration, and it ig 

 in such cases that the pouch is punctured. This membrane receives 

 numerous fine vascular and nervous ramifications from the neighbouring 

 branches. 



" The guttural pouches communicate with the pharynx and cavity of the 

 tympanum, and usually contain air ; the quantity of this may vary in health, 

 according to the degree of dilatation of these membranous sacs. Their 

 dilatation is chiefly produced by the palato-pharyngeus muscle, several 

 fibres from which extend to their mucous membrane ; and, besides, when 

 the ear is erected this membrane is thrown into a state of tension, through 

 the adhesion of the lower prolongation of the concha to its surface. 



" The functions of the guttural pouches are far from being known. It 

 cannot be affirmed that they increase phonation ; indeed, their use appears 

 to be rather related to audition, if it be considered that these annexes of the 

 guttural duct of the tympanum coincide, in Solipeds, with a less development 

 of the mastoid cells than in the other animals. 



