28 
THE DOG'S SKULL. 
ing a saddle-shaped head. Posteriorly 
a process extends on to the periotic 
bone, in a fossa of which it articulates. 
The incus is pyriform in shape; the 
two processes forming the upper por- 
tion of the bone; the lower portion or 
stalk bends downwards and inwards 
and becomes attached to a minute 
disc-like bone, the os orbiculare. 
y. The stapes is stirrup-shaped: ventrally 
its base is attached to the membrane 
covering the fenestra ovalis ; while its 
upper portion, which forms the arch, 
articulates with the os orbiculare. 
With regard to the development and homologies of 
the auditory ossicles, much uncertainty prevails. 
The generally accepted views are as follows. 
The malleus and incus arise in connection with 
the mandibular arch, the former from the lower 
portion and the latter from the upper portion 
of the arch; the malleus being homologous 
with the articular and angular elements of 
Meckel’s cartilage, and the incus with the palato- 
quadrate cartilage of lower Vertebrata. The 
stapes arises in connection with the hyoid arch, 
and an ossification from the periotic capsule ; 
according to Kitchen Parker, Hertwig and 
others, it is the homologue of the columella in 
Birds, Reptiles, and Amphibia. 
