THE SKIN AND SENSE ORGANS 295 



out from the walls of the ear capsule. The former is the functional 

 element in the larva, and the latter in the adult. 



The columella develops in a ligament, the suspensor-stapedial liga- 

 ment^ stretching from the membrane of the fenestra to the proximal 

 end of the quadrate and squamosal. It consists of two portions — first 

 a short rod-like stylus connecting the suspensorium (squamosal and 

 quadrate) with the second portion — a flat plate-like expansion filling 

 the anterior portion of the fenestra. In the adult Salamander the 

 columella is ossified and fuses with the wall of the auditory capsule 

 so as to be scarcely distinguishable from it. 



The operculum (fig. 2, cop.) fills the remainder of the fenestra 

 vestibuli. It is a plate of cartilage, approximately circular in plan, and 

 shaped like a very thick crescent in transverse section. It is attached 

 to the auditory capsule at its edges by the membrane of the fenestra, 

 and thus lies entirely free from other skeletal parts, being supported 

 by an elastic membrane only. As already mentioned it develops from 

 the wall of the ear capsule, but it is perhaps questionable whether it 

 is 'cut out', as it were, by absorption of the tissue round it, or whether 

 it arises by an extension of cartilage into the membrane closing the 

 fenestra. It appears probable that the latter is the chief process 

 at work. The operculum is attached to the supra-scapula by a strap- 

 like muscle derived from the M. levator scapulae, and called by Gaupp 

 M. opercularis. The presence of this muscle is at first sight rather 

 extraordinary, and it is absent in those forms, e.g. Necturus, which 

 lead an entirely aquatic existence in both larval and adult phases. In 

 such forms also the columella \s free from the auditory capsule but 

 fused with the operculum, and it is supposed that in aquatic forms, 

 both larvae and adults, the auditory stimuli reach the ear from the 

 external medium — water — by means of the following path : — jaws, 

 suspensorium, columella, fenestra. Such forms, particularly when 

 adult, are much given to resting the whole of the body, including the 

 jaws, on the bottom, and the lower jaw is thus in a favourable position 

 for receiving vibrations directly from the ground. Now in terrestrial 

 forms, e.g. Salamandra, the head is always kept well clear of the 

 ground, and, since it is manifestly impossible for the vibrations of a 

 rare medium like air to affect so heavy a skeletal structure as the 

 jaws, the auditory stimulus must travel by some alternative route in 

 these animals. This route is by means of the fore-limbs to the scapula 

 and suprascapula, and from thence to the operculum via the M. oper- 

 cularis. In this connexion it is interesting to note that Dunn (1922) 

 gives a description of a primitive Urodele — Hynobius leechi — in which 

 the columella is free from the auditory capsule, and there is in 



