The incus is articulated with the quadrate end of the mandibular arch, and its rounded head comes in contact with the stapes ([fig. 342], st) which is segmented from the fenestra ovalis. The main arch of the hyoid becomes divided into a hypohyal (h.h) below and a stylohyal (st.h) above, and also becomes articulated with the basal element of the arch behind (bh).
In the course of further development the Meckelian part of the mandibular arch becomes enveloped in a superficial ossification forming the dentary. Its upper end, adjoining the quadrate region, becomes calcified and then absorbed, and its lower, with the exception of the extreme point, is ossified and subsequently incorporated in the dentary.
The quadrate region remains relatively stationary in growth as compared with the adjacent parts of the skull, and finally ossifies to form the malleus bone of the ear. The processus gracilis of the malleus is the primitive continuation into Meckel’s cartilage.
The malleus and incus are at first embedded in the connective tissue adjoining the tympanic cavity (hyomandibular cleft, vide p. [528]); and externally to them a bone known as the tympanic bone becomes developed so that they become placed between the tympanic bone and the periotic capsule. In late fœtal life they become transported completely within the tympanic cavity, though covered by a reflection of the tympanic mucous membrane.
The dorsal end of the part of the hyoid separated from the incus becomes ossified as the tympano-hyal, and is anchylosed with the adjacent parts of the periotic capsule. The middle part of the bar just outside the skull forms the stylohyal (styloid process in Man) which is attached by ligament to the anterior cornu of the hyoid (ceratohyal).
While the account of the formation of the malleus, incus, and stapes just given is that usually accepted in this country, a somewhat different view of the development of these parts has as a rule been adopted in Germany. Reichert (No. [461]) held that both the malleus and the incus were derived from the mandibular bar; and this view has been confirmed by Günther, Kölliker and other observers, and has recently been adopted by Salensky (No. [462]) after a careful research especially directed towards this point. Reichert also held that the stapes was derived from the hyoid bar; but, though his observations on this point have been very widely accepted, they have not met with such universal recognition as his views on the origin of the malleus and incus. Salensky has recently arrived at a view, which is in accord with that of Parker, in so far as the independence of the stapes of both the hyoid and mandibular arches is concerned. Salensky however holds that it is formed from a mass of mesoblast surrounding the artery of the mandibular arch, and that the form of the stapes is due to its perforation by the mandibular artery. A product of this artery permanently perforates the stapes in a few Mammalia, though in the majority it atrophies.
In view of the different accounts of the origin of the incus the exact nature of this bone must still be considered as an open question, but should Reichert’s view be confirmed the identification of the incus with the columella of the Amphibia and Sauropsida must be abandoned.
Membrane bones and ossifications of the cranium.
The membrane bones of the skull may be divided into two classes, viz. (1) those derived from dermal osseous plates, which as explained above (p. [542]) are primitively formed by the coalescence of the osseous plates of scales; and (2) those formed by the coalescence of the osseous plates of teeth lining the oral cavity. Some of the bones sheathing the edge of the mouth have been formed partly by the one process and partly by the other.
In the Fishes there are found all grades of transition between simple dermal scutes, and true subdermal osseous plates forming an integral part of the internal skeleton. Dermal scutes are best represented in Acipenser and some Siluroid Fishes.