THE SKULLS OF MAMMALIA. 



249 



Fig. 99. 



I have already endeavoured to show that the quadrate and 

 articular bones of the oviparous Vertebrata are represented by 

 the incus and malleus of Man, and consequently by the corre- 

 sponding bones in all Mammalia ; and that, as a consequence of 

 the appropriation of two bones of the mandibular series to the 

 uses of the organ of hearing, the dentary bone develops its 

 own condyle, and articulates with the squamosal. 



Another bone which appears to have no distinct representa- 

 tive in most oviparous Vertebrates* is very conspicuous in most 

 Mammals, and far more so in the Beaver than in Man. This 

 is the tympanic element, and it will be useful to study with 

 especial attention the characters of this bone, its relations to the 

 periotic, and the manner in which both are connected with the 

 other bones of the skull. 



In a transverse section of the conjoined tympanic and periotic 

 bones, taken through the canal which is common to the anterior 

 and posterior vertical semicircular 

 canals (Fig. 99), the periotic mass 

 is seen to be prolonged, external to 

 and below the horizontal semicircular 

 canal and that for the passage of the 

 portio dura, into a stout " mastoid 

 process " (If), which appears upon the 

 outer surface of the skull, between the 

 ex-occipital, the squamosal, and the 

 tympanic, as a production downwards 

 and outwards of the "pars mastoidea" 

 which is doubtless, as in Man, com- 

 posed partly of the pro-otic and partly 

 of the epiotic and opisthotic bones. 



The tympanic bone is produced, externally, into a spout-like 

 tube, directed forwards and upwards, which is the external audi- 

 tory meatus (Au, Fig. 98) ; below and internally the tube dilates 

 into a thin walled hemispherical bulla (b, c, Fig. 99), open su- 

 periorly, and produced in front and anteriorly into a perforated 

 process, which contains the osseous part of the Eustachian tube. 



* I learn from Mr. Parker that all Birds above the Strutliionidse have a more or 

 less perfect chain of tympanic bones, of which there are six in Corvus corone. 



En 



Ficr. 99. — A vertical section of the 

 conjoined tympanic and periotic 

 bones of the Beaver (Castor fiber) . 

 a, external auditory meatus ; b, 

 groove for the tympanic mem- 

 brane ; c, the inner lip of the 

 tympanic ; Eu, Eustachian tube ; 

 CI, cochlea ; M, pars mastoidea. 



