PLATAN 1ST A. 453 



as valves. The Eustacliian tube, in the adult, at its termination has a diameter of 

 0"40 inch, and the guttural pouch is half an inch broad at its beginning. From the 

 crescentic fold the direction of the guttural pouch is downwards, backwards, and 

 inwards, and the postnarial passage Lies on the commencement of the outer wall of 

 the pouch, the two lying back to back ; hence the difficidty of passing a probe 

 from the guttural pouch into the posterior nares, for, when it reaches the orifice 

 above the crescentic fold, it meets with the outer wall of the external passage, and 

 a similar difficulty is encountered in tracing the Eustachian canal from its auditory 

 end. 



As these prolongations of the Eustachian tube are undoubtedly air receptacles, 

 and in all probability subservient to audition, as seems to be the function of their 

 homologues in the horse, we have an explanation of the position, direction, and 

 valvular character of the external orifice in an animal which has to breathe by 

 rising to the surface of water. 



When the Eustachian tube reaches the outer wall of the posterior nares, it 

 dilates into a tube of considerable capacity, being half an inch in diameter (m 

 an individual 4| feet long) at the point where it opens into the posterior nares, 

 and inunediately behind this, it suddenly expands into a large sac, lying internal 

 to the stylohyoid, to which its outer wall is attached. Turning round that bone 

 from behind forwards, tliis sac lies between it and the thyrohyal, and its internal 

 wall lies against the outer wall of the back of the pharynx, while the roof of 

 the sac lies below the exoccipital and basi-occipital bones. In the same individual, 

 it has a length from before backwards of 1'75 inch, and a vertical height of 

 1-25 inch. The inner surface is white smooth glistening and tendinous in appear- 

 ance, and its wall has numerous deep recesses of various dimensions formed by 

 archino- folds of the raembrane constituting the walls of the sac ; while other parts 

 of it are covered by a tendinous meshwork, defining shallow crypts, of the same 

 structure as those found on the walls of the posterior nares. 



Some of the recesses lead into small secondary pouches, and from these into a 

 labyrinth of smaller passages. A rather wide orifice, lying at the posterior margin 

 of the stylohyoid, leads into a pouch divided into crypts and little pockets by valvular 

 folds which turn round the outer margin of that bone, and lie along the cranial 

 end of the thyrohyal. About the middle of the great sac from which this little pouch 

 is given off, a strong valvular fold arches across it from the external to the internal 

 walls, and serves to close a wide orifice leading into a second great sac hereafter to 

 be considered. Behind this valve, and at the bottom of the sac, there are seven 

 small orifices arranged in Hnear series, and each separated from its feUow by a pillar- 

 like fold; and immediately above these occurs the opening leading into the se- 

 condary sac just described. These orifices, wliich, however, vary in number in 

 different individuals, when traced out are found to lead into a third secondary sac, 

 which passes downwards behind the thyrohyal. This orifice leading from the 

 strong valvular fold just mentioned opens into the secondary sac, which is divided into 

 compartments by large valves, and communicating with the sac behind the thyro- 



