CHELONE MYDAS. 355 



ligamentous substance is also convex externally, and is a covering to 

 the larger part of the external cavity of the ear, which cavity is con- 

 tinued backward, like the cells of the mastoid process in other animals. 

 The oblong body [representing the membrana tympani] is convex 

 externally and concave internally, in shape something like the mouth 

 of a spoon : it is loosely lined with a fine transparent membrane, which 

 seems to be a covering for that part of the ear, and is attached to it by 

 pretty long fibres. 



The Eustachian tube begins at the posterior part of the mouth, a 

 little behind the articulation of the lower jaw. It is membranous at 

 the beginning, but soon becomes ligamentous on the posterior part, and 

 bony on the anterior, excepting just at the middle, where there is a 

 little piece of cartilage, which makes this part a little more bent. The 

 membranous part is a little contorted, and becomes a little narrower at 

 the beginning of the cartilage. The bony and cartilaginous part forms 

 one curve, whose convex part is turned backward. The cartilage takes 

 the same sweep with the bone, and seems to fit it ; so that the cartilage 

 seems, in its natural situation, to lay close to the bone, which flattens 

 the canal. This cartilage is moveable, and projects a little into the 

 cavity of the ear ; it has a very strong muscle attached to the posterior 

 convex surface about the middle, which pulls it back, and at the same 

 time a little outward, from the cavity of the ear. This enlarges the 

 cavity of the canal, and by bringing out the cartilage it seems to have 

 some effect upon the bone of the ear by being attached to it 1 . 



The other part of the ear is enclosed chiefly in bone, which is much 

 harder where it makes the inner surface of the [acoustic] cavities than 

 elsewhere. That part which is next to the brain, or that which makes 

 part of the cavity of the skull, is cartilaginous. 



Under the horny ligamentous substance before mentioned [the ear- 

 drum, or homologue of the membrana tympani], is a large oblong 

 cavity, which answers to the tympanum in the human. Its direction is 

 forwards and backwards ; it is one circumscribed cavity lined with a 

 dark-coloured membrane, having the Eustachian tube opening on the 

 external end. Erom the concave side of the horny [ear-drum] passes 

 a long small bone to the vestibulum : in its passage through the tym- 

 panum it is attached by its whole length to the inner end of the carti- 

 lage of the Eustachian tube. Then it enters a bony canal, surrounded 

 by a strong ligamentous substance, but is only loosely attached to this 

 substance, so as to have free motion in it. At its inner end it becomes 



1 [Both the tensor and laxator tympani arise from the cartilaginous part of the 

 Eustachian tube in mammals, and are inserted into the malleus.] 



2 A 2 



