152 



LECTURE VI. 



admission and appreciation of these pre-ordained adaptations to 

 special exigencies in the skeleton of Fishes, the superior strength and 

 complex development of the tympanic pedicle and its appendages 

 would be inexplicable and unintelligible in this lowest and iirst-born 

 class of Vertebrate animals. 



In contrasting the skeletons of the Fish and Mammal, with refer- 

 ence to hypothetical secondary origins of organic species ; such, for 

 examjjle, as that of transmutation and progressive ascent of specific 

 forms ; the vast disparity of the hyoidean arch, in point of size, com- 

 plexity, and strength, both intrinsic, and as due to its connections, 

 must not be overlooked. Its small size, simple structure, and loose 

 suspension in the flesh, have led to its being reckoned in Anthro- 

 potomy as a single bone ; and it is rarely preserved in the artificial 

 skeletons of man or beast : whilst if absolute and relative magnitude, 

 complexity of structure, arid importance of function, are tests of the 

 grade of organisation of a part, the progress of development must be 

 held to have been reversed in respect of the hyoid arch ; which, with 

 its appendages, offers the highest grade in Fish and the lowest in 

 Man. And why this great difference — this striking exception to the 

 general condition of the ichthyic organisation? It is explicable only 

 on teleological principles. It is true the Fish tastes not with its 

 tongue, neither does it speak : the sole function of the human tongue- 

 bone, which is performed by that of the fish, is that which is in subser- 

 viency to deglutition. But this function is not in relation to food 

 alone ; all the mechanical part of breathing in the fish is a modified 

 act of swallowing. The hyoid arch is the chief point of suspension of 

 the visceral arches which support the gills ; and the branchiostegal 

 membranes, stretched out upon the diverging rays of the hyoid arch, 

 regulate the course and exit of the respiratory currents : thus the 

 mechanical functions of the thorax of the air-breathing classes are 

 transferred to the hyoid arch and its appendages in Fishes. 



By the retraction of the hyoid arch the opercular doors are forced 

 open, and the branchial cavity is widened ; whilst all entry from be- 

 hind is prevented by the branchiostegal membranes, which close the 

 posterior branchial slits : the water, therefore, enters by the gaping 

 mouth, and rushes through the sieve-like interspaces of the branchial 

 arches into the branchial cavity : the mouth then shuts, the opercular 

 doors press upon the branchial and hyoid arches, which again advance 

 forwards, and the branchiostegal membranes being withdrawn, the 

 cui'rents rush out at the open jiosterior branchial orifices. These 

 functions are the true condition of the higli development of the os 

 hyoides in fishes. 



I have noticed the great development, the persistence, and ossifi- 



