THE SENSE ORGANS 151 
We have thus, in each case, a typical example of change of 
function. 
WHY: 
YWYGE 
WZ 
©) 
EN 
Fic. 94.—DIAGRAM TO ILLUSTRATE THE METAMORPHOSIS DURING DEVELOPMENT OF 
(I-V) THE First TO THE FIFTH VISCERAL SKELETAL ARCHES. 
From the first arch (the so-called Meckel’s cartilage) two of the auditory ossicles, the 
malleus and the incus (md. and in.), are represented as arising proximally, but about 
this there is still considerable doubt (cf. ante, p. 64). ., pinna; st., stapes; pr., 
processus mastoideus of skull. 
From the second (hyoid) arch arise, proximally, the processus styloideus (p.s.), 
distally the anterior or lesser cornua of the hyoid (c.a.), and a portion of the basi- 
hyoid or copula (ds.). By far the greater portion of this arch becomes the stylo- 
hyoid ligament (/g.). It is very doubtful whether the arch of the stapes also arises 
from the proximal portion of the second arch ; the basal plate of the stapes, at any 
rate, appears to arise independently of it. . 
The third arch gives rise to the greater part of the body (ds.), and the posterior 
or greater horn, of the hyoid (c.p.). 
The fourth arch gives rise to the upper segment (th’.) of the thyroid cartilage 
and the fifth to the lower one (th”.). The arytenoid cartilage (a.7.) is probably a deri- 
vative of the fifth arch. ¢c., the cartilago triticea ; c7., cricoid cartilago; ¢r., trachea. 
proved its regular occurrence, in a modified form, throughout the living Ganoids ; 
and further, that in these fishes and certain Selachians it gives off a diverticulum 
(the canalis tubo-tympanicus), which there is reason to regard as the possible homo- 
logue of the middle auditory chamber of the terrestrial Vertebrata (cf. Ramsay 
Wright, Jour. Anat. and Phys., vol. xix. p. 476). ] 
