THE SKELETON OF THE DOG. THE SKULL. 393 



periotic by the tympanic cavity into which the fenestra 

 rotunda and the fenestra ovalis open. 



There are several other openings into the tympanic cavity. 



(a) On the external surface is a large oval opening, the 

 external auditory meatus bounded by a thickened rim. 



(6) Into the outer and anterior part of the cavity the 

 outer end of the Eustachian tube opens; while the inner 

 end passes through a foramen (fig. 75, 22) just external to the 

 foramen lacerum medium, on its way to open into the pharynx. 



(c) The internal carotid artery also enters the tympanic 

 cavity by a canal which commences in the foramen lacerum 

 posterius, and passes forwards to open on the inner side of the 

 bulla. The artery then passes forwards, and barely appearing 

 on the ventral surface of the cranium, enters the brain cavity 

 through the foramen lacerum medium (fig. 75, 9). 



Immediately behind the tympanic, between it and the 

 mastoid process of the periotic and the paroccipital process of 

 the exoccipital is the stylomastoid foramen (fig. 75, VIII). 



Within the tympanic cavity are four small bones, the 

 auditory ossicles (cp. fig. 74), called respectively the mal- 

 leus, incus, lenticular and stapes; these together form a 

 chain extending from the fenestra ovalis to the tympanic 

 membrane. 



The malleus has a somewhat rounded head (fig. 100, B, 1) 

 which articulates with the incus, while the other end of the 

 bone is drawn out into a long process, the manubrium, 

 which lies in relation to the tympanic membrane. The head 

 is also more or less connected by a thin plate of bone, the 

 lamella, to another outgrowth, the processus longus. The 

 incus (fig. 100, B, 3) is somewhat anvil-shaped, and is drawn 

 out into a process which is connected with the lenticular, a 

 nodule of bone interposed between the incus and the stapes, 

 with which it early becomes united. The stapes (fig. 100, B, 2) 

 is stirrup-shaped, consisting of a basal portion from which 

 arise two crura, which meet and enclose a space, the canal. 



