| OCR Text |
Show 1869.] THE MALLEUS AND THE INCUS OF THE MAMMALIA. 401 is sufficient to prove that the inner extremity of i must needs lie beneath and internal to the eye, and cannot by any possibility come near the fenestra ovalis. It therefore seems to be impossible that i can be the stapes. Bearing clearly in mind the demonstration now given that the stapedial apparatus (if I may so term the stapes with its appendages) of the Sauropsida is connected entirely with the hyoidean arch, and that it consists of a stem terminating, at one end, in the plate which covers the fenestra ovalis, and, at the other, in sundry processes of cartilaginous or fibrous texture, one of which is connected with the tympanic membrane (when that structure exists), while another passes up to be united with the otic region of the skull, close to the articulation of the quadrate bone, we may pass to the consideration of the homologies of these parts in the ordinary Mammalia, of which Mari may be taken as an example. The Okenian view, adopted by Prof. Peters, assumes that the ramus of the mandible of the Mammal answers to the whole ramus of the mandible of a Sauropsidan, that the tympanic bone of the Mammal answers to the quadrate bone of the Sauropsidan, and that the ossicula auditus of the Mammal, or the malleus, incus, and stapes, collectively, correspond with the stapedial apparatus of the Sauropsidan. The Reichertian view, which I have hitherto supported, assumes that the ramus of the mandible of the Mammal answers only to part of the ramus of the Sauropsidan, inasmuch as the articular piece of the Sauropsidan mandible answers to the malleus of the Mammal-that the quadrate bone of the Sauropsidan is the homologue of the incus of the Mammal-and, consequently, that the stapedial apparatus of the Sauropsidan is entirely represented bv the stapes of the mammal. In the place of the tympanic bone of the mammal there are only the ossifications which are found in the membranous frame of the tympanic membrane in some Sauropsida (e.g. many birds) and Amphibia. The arguments by which this view has been supported are briefly these :•- In the Sauropsidan embryo a rod of cartilage occupies the first visceral arch on each side, and meets its fellow in the middle line. The rod becomes jointed, and the part on the distal side of the joint is converted into Meckel's cartilage, while that on the proximal side of the joint is modelled into the rudiment of the quadrate bone, which is invariably, in its earliest state, cartilaginous. Soon, however, the quadrate cartilage ossifies, and a centre of ossification appears in that part of Meckel's cartilage which articulates with the quadratum. This gives rise to the articular element of the mandible. All the other constituents of the lower jaw are developed in the fibrous tissue which surrounds the rest of Meckel's cartilage, which structure either persists throughout life, or disappears. In a mammalian embryo the first visceral arch also contains a rod of cartilage, which, there can be no doubt, is the homologue of that in the Sauropsidan. The ramus of the mandible is developed in the fibrous tissue which surrounds the distal portion of the rod, which |
