THE STRUCTURE OF THE TIKE'S SKULL. 170 



Tin's rod of cartilage affords a safe basis upon which to found 

 a homological argumentation. For it most certainly corresponds 

 with Meckel's cartilage in the human foetus, and the dentary 

 bone lies outside it, in just the same way as the dentigerous 

 ramus of the human mandible lies outside Meckel's cartilage. 

 But the articular bone is an ossification in and around the proxi- 

 mal end of Meckel's cartilage in the Pike, just as the malleus is 

 an ossification in and around the proximal end of Meckel's car- 

 tilage in the human foetus ; and the os quadratum is related to 

 the os articular e of the fish in the same way as the incus is 

 related to the malleus. 



Hence it is to be concluded, in the absence of any evidence 

 to the contrary, that the articular piece of the Pike's lower jaw 

 answers to the malleus, and the quadrate bone to the incus. 



I am not aware that any evidence can be adduced against 

 this view ; but, on the other hand, the relations of the parts 

 thus identified to the portio dura of the seventh nerve, in Man 

 and in the Fish, seem to me to afford it much support. 



The portio dura in the former perforates the pars petrosa, 

 and, after skirting the inner wall of the tympanum, external to 

 the labyrinth, leaves the skull by the stylo-mastoid foramen. 

 Before it does so, however, it gives off a recurrent branch, the 

 chorda tijmpani, which takes a very singular course — passing 

 between the pyramid, which is the upper end of the hyoidean 

 arch, and the tympanic bone, entering the tympanum, crossing 

 the auditory ossicles to make its way out at the front wall of the 

 tympanum, between the tympanic and the squamosal, then 

 uniting with the gustatory division of the trigeminal, and passing 

 down along the inner side of the ramus of the mandible with it, 

 until eventually it leaves it to become connected with the sub- 

 maxillary ganglion. 



The principal portion of the portio dura, on the other hand, 

 makes its way out by the stylo-mastoid foramen, and is dis- 

 tributed to the facial muscles, some comparatively insignificant 

 branches only, being furnished to the levators of the hyoidean 

 apparatus and depressors of the low r er jaw. But, as has been 

 already stated, the facial muscles, so important and largely de- 

 veloped in Man, become insignificant in the lower Vertebrates, 



n 2 



