486 MEMBRANE BONES. 



ity, though covered by a reflection of the tympanic mucous mem- 

 brane. 



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 stylo-hyal (styloid process in Man) 

 which is attached by ligament to the anterior cornu of the hyoid 

 (cerato-hyal). 



While the account of the formation of the malleus, incus, and stapes 

 just given is that usually accepted in this country, a somewhat different 

 \'iew of the development of these parts has as a rule been adopted in 

 Germany. Eeichert (No. 461) held that both the malleus and the incus 

 were derived from the mandibular bar ; and this vi6w has been confirmed 

 by Giinther, Ktilliker 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 by fid 

 bar ; but, though his obsei'vations 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 indepen- 

 dence 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 pei'manently 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 hones 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. 447) 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 tliere are found all grades of transition between 

 simple dermal scutes, and true subdermal osseous phites forming an 

 integral part of the internal skeleton. Dermal scutes are best repre- 

 sented in Acipenser and some Siluroid Fishes. 



Where the membrane bones still retain the character of dermal 

 plates, those on the dorsal surface of the cranium are usually arranged 

 in a series of longitudinal rows, continuing in the region of the head 

 the rows of dermal scutes of the trunk ; while the remaining cranial 

 scutes are connected with the visceral arches, Tiie dermal bones 

 on the dorsal surface of the head are very different in number, 



