358 Comparative Anatomy — Its History, Aim, and Method 



small aperture in the thin wall between the tympanic cavity and the 

 space within which the inner ear lies. This chain transmits vibrations 

 from the tympanic membrane to the fluid which fills the thin space 

 between the wall of the sensory internal ear and the bone surrounding 

 it. 



Comparing the shark's breathing organs with the mammal's audi- 

 tory mechanism, three points of resemblance are noted. The shark's 

 spiracle opens on the surface of the head at about the place where an ex- 

 ternal ear might be expected, if the shark had one. Secondly, the spiracle 

 leads from the pharynx to the exterior, and so would the Eusta- 

 chian tube lead from the pharynx to the exterior by way of the 

 external auditory canal (meatus), if the passage were not closed by the 

 tympanic membrane. Thirdly, the sensory ear of the shark, a nervous 

 structure resembling the "inner" ear of the mammal, but much simpler, 

 is enclosed within a lateral projection of the posterior region of the 

 cartilaginous cranium (Fig. 115C), and the spiracular cavity is situated 

 very close to the wall of this auditory region of the cranium, just as the 

 tympanic cavity is close to the inner ear of the mammal. No other 

 resemblances appear in the adult organs — but now we recall that paired 

 pharyngeal pouches, usually piercing to the exterior, are present in all 

 vertebrate embryos. 



In the dogfish embryo there are six pairs of pharyngeal pouches, all 

 opening externally. Those of the most anterior pair become the spi- 

 racles of the adult. The others become the five pairs of gill-chambers. In 

 mammals the embryonic pharyngeal pouches ordinarily do not pene- 

 trate to the exterior and they are soon completely obliterated, except 

 those of the most anterior pair. These pouches, judged by their relations 

 to neighboring embryonic parts, clearly correspond to those which, in 

 the shark, perforate and form spiracles. In the mammalian embryo 

 each of these pouches extends toward the lateral surface of the head 

 and almost perforates, but remains closed by a thin membrane consist- 

 ing of the ingrowing external layer (ectoderm) of the embryo closely 

 joined with the outgrowing wall (endoderm) of the pharynx. This 

 double layer persists as the tympanic membrane. In later development 

 the pouch becomes elongated and changed in form, appearing in the 

 adult as the tympanic cavity and Eustachian tube. The three small 

 ear-bones of the mammal arise from embryonic parts corresponding 

 to those which, in "the embryos of fishes, give rise to certain skeletal 

 elements of the jaw mechanism (Fig. 520). 



Comparison of the breathing mechanism of a shark with the audi- 

 tory mechanism of a mammal leaves us in some perplexity. Viewing the 

 two mechanisms as wholes, there would seem to be no correspondence 

 between them. There is certainly none as to function. Resolving the 



