498 MAMMALIA. 



The auditory organ differs from that of the Sauropsida prin- 

 cipally in the greater development of the external auditory 

 meatus and of the external ear (pinna), in the greater number of 

 sound-conducting bones (malleus, incus, stapes), in the presence 

 of the organ of Corti, and except in Monotremata in the spiral 

 winding of the cochlea and the absence of a papilla acustica 

 lagenae. The ductus endolymphaticus proceeds from the narrow 

 canal connecting the saccule and the utricle ; it perforates the 

 periotic bone, enters the cranial cavity and ends in the dura mater 

 in a small dilatation, the saccus endolymphaticus. The auditory 

 nerve has six terminations, the papilla acustica lagenae and the 

 macula neglecta being absent. The windings of the cochlea vary 

 in number from IJin Erinaceus europaeus to 5 in Coelogenys 

 paca. 



The tympanic cavity is more spacious than in the lower forms, 

 being frequently swollen into the bulla ossea which is formed 

 by the alisphenoid in the Marsupialia, and by the os tympanicum 

 in other forms. It communicates with the pharynx by a wide 

 opening in Monotremata, but in other Mammalia there is along 

 eustachian tube. It is also in communication with cavities 

 in the adjacent bones (air-cells of the mastoid, etc.). 



The stapes is usually perforated, but it is columelliform and 

 unperf orated in Monotremata, Perameles, Manis and some other 

 Mammalia. The homologies of the mammalian auditory ossicles 

 have been much disputed'.* By Huxley the stapes was regarded 

 as the columella auris of the Sauropsida and the incus as the 

 suprastapedial part of the same structure (see especially Spheno- 

 dori). On this view the malleus is the homologue of the quad- 

 rate. Others regard the malleus as the os articulare, the incus 

 a-s the quadrate and the stapes as the columella. Finally it is 

 held by some anatomists that the whole chain of ossicles is 

 comparable to the columella auris and its various processes of 

 the Sauropsida, the quadrate appearing in mammals as the os 

 tympanicum. 



Huxley's view is based largely upon the arrangement of the parts in 

 Sphenodon, and upon the fact that in the mammalian embryo, the pro- 

 cessus gracilis of the malleus is continuous with Meckel's cartilage. 



* Huxley, On the representatives of the malleus and incus of the Mam- 

 malia in the other Vertebrata, P. Z. S., 1869. Gadow, " On the modifi- 

 cations of the first and second visceral arches, " Phil. Trans., 179. 



