494 HISTORY OF THE HUMAN BODY 



pouch of the spiracular opening, the one associated, as will be 

 remembered, with the hyoid arch. The inner portion of this 

 cavity communicates with the pharynx and forms the auditory 

 or Eustachian tube, but direct communication with the outside 

 is prevented by the presence of a circular tympanic membrane 

 at the outer end, just beneath the skin, and usually very evi- 

 dent from the outside. 



This membrane, which is covered outwardly by integument 

 and on its inner side by mucous membrane, is a separate 

 formation, usually of connective tissue, but in a few cases it is 

 cartilaginous. It has been doubtfully homologized with the 

 spiracular cartilage of selachians, but this is too uncertain to 

 be definitely asserted. In many cases there exists a second 

 opening in the wall of the otic capsule, the fenestra cochleae 

 [rotunda], filled with a thin membrane, also termed the inner 

 tympanic membrane. This part is present in all higher verte- 

 brates, thus giving the tympanum the characteristic from 

 which it derives its name, i.e., two drum heads, outer and 

 inner. To complete the likeness of the middle ear to a drum 

 the Eustachian tube represents the opening always present in 

 the cylinder of a drum and employed in both purposes for 

 equalizing the air pressure on either side of the drum heads. 



A mechanism, however, which is lacking in a drum, is that 

 formed by the columella, a delicate spindle of bone or cartilage, 

 which extends from the center of the outer tympanic mem- 

 brane to the operculum. By this means the sound vibrations 

 that impinge upon the former are transmitted directly to the 

 latter, and through it to the perilymph within the otic capsule. 

 Another channel for the transmission of sound waves is fur- 

 nished by the air enclosed in the tympanic cavity, the vibrations 

 striking the inner tympanic membrane. The apparently new 

 skeletal element, the columella, is probably nothing more than 

 a process of the operculum, but it has been considered by some 

 to be a distinct element and to represent the hyo-mandibular 

 of fishes, employed there as a suspensory piece for the man- 

 dible and originally the dorsal segment of the second visceral 

 arch (hyoid). 



