42 On the Development of the Mouth and Tongue of the Frog. 



The increase in the number of the Fig. 4. 



nen^e-tubes in each fungiform papilla, Masnified 400 diameters, 

 with the age of the animal, must neces- 

 sarily be accompanied with a correspond- 

 ing increase in the number of tubes 

 composing the trunk of the glossopharyn- 

 geal, from whence all the fibresare derived; 

 for in the nervous trunks the indepen- 

 dence of the neiTC-tubes is an established 

 fact, confirmed by every-day observation. 

 In the substance of the tongue we can 

 so easily trace the peripheric end of the 

 nerve up towards the trunk for so consi- 

 derable a distance, that no subdivision of 

 the primitive fibres can be regarded as in '"Trot'^igt^gfiTacZ""'" 

 the least degree probable in this part of 



their course. The same increase of fibres is likewise observed in 

 the mammiferse ; the tubes are too indistinct and too close toge- 

 ther to allow us to determine numerically their difference at dif- 

 ferent ages. 



It is probable that the increase of neiTC-tubes is not confined 

 to the neiTcs supplying the tongue, but is a general law appli- 

 cable to the whole neiTOus system, as we obsei-ve that, with the 

 progress of age, the rest of the nervous trunks undergo an aug- 

 mentation in size in the same manner as the glossophai-yngeal. 



In the frog, their augmentation in number, from the time at 

 which they are first distinctly seen to that at which the animal 

 is full gi'own, is from two to eight. Supposing this to be the 

 general ratio of increase for the whole system, we see how great 

 umst be the difference in the nervous powers at diff'erent ages. 



The other characteristics of the young nei-ve-tubes are their 

 comparative minuteness, being at first not more than half the 

 size they ultimately attain, the general absence of the white 

 substance, and their uneven varicose shape, not unlike that of the 

 white cerebral fibres. The same minuteness of the tubular fibre 

 is seen throughout the nerve up to the brain. In a young frog 

 weighing 15 grains, their maximum size was about ^^'^^dth of 

 an inch. In the adult the same nerve-tubes were 20V0' ^^^ ^^ 

 the sciatic of the same animal they were yj^no^^*^ *^^ ^^^ inch. 



The alteration in the vascular system durmg the various stages 

 of development of the papillary organs is not less manifest than 

 that of the nen'cs. In the first stage the papilla receives a nar- 

 row capillaiy, which forms a close bend on itself on reaching the 

 point of insertion of the papilla. With the inci-ease of the organ 

 the vessel quickly shows a double loop or bend, giving it a 

 crutched appearance ; while in the adult animal there is found 

 an increase in the size of the vessels, and frequently an increase 



