248 



Mr. H. Mance on the Measurement 



[Jan. 19, 



definite patches ; these are the cranial roof-bones. Around the mouth 

 there are cartilages like those of the Lamprey and the Chimsera ; but 

 these yield in interest to the propfr facial bars, which are as follows, 

 namely : — 



First pair, the " trabecule." 



Second pair, the mandibular arch. 



Third pair, the hyoid arch. 



And fourth to seventh pairs : these are the branchials. 



These are all originally separate pairs of cartilaginous rods ; and from 

 these are developed all the complex structures of the mouth, palate, face, 

 and throat. The pterygo-palatine arcade is merely a secondary connecting 

 bar developed, after some time, between the first and second arches. 



Meckel's cartilage arises as a segmentary bud from the lower part of the 

 second, and the " stylo-cerato-hyal," as a similar secondary segment, 

 from the third arch. 



By far the greater part of the cranium (its anterior two-thirds) is deve- 

 loped by out-growing laminae from the trabecular, which after a time be- 

 come fused with the posterior or vertebral part of the skull. 



When the tadpole is becoming a frog, the hyoid arch undergoes a truly 

 wonderful amount of metamorphosis. 



The upper part, answering to the hyomandibular of the fish (not to the 

 whole of it, but to its upper half), becomes the " incus," and a detached 

 segment becomes the " orbiculare," which wedges itself between the incus 

 and the "stapes." The stapes is a "bung" cut out of the " ear-sac." 

 The stylo-cerato-hyal is set free, rises higher and higher, and then articu- 

 lates with the " opisthotic " region of the ear-sac ; in the toad it coalesces 

 therewith, as in the mammal. The lower part of the hyomandibular coa- 

 lesces with the back of the pair of the mandibular arch ; and the " sym- 

 plectic " of the osseous fish appears whilst the tadpole is acquiring its limbs 

 and its lungs, and then melts back again into the arch in front ; it is re- 

 presented, however, in the bull-frog, but not in the common species, 

 by a distinct bone. 



This very rough and imperfect abstract must serve at present to in- 

 dicate what has been seen and worked out in this most instructive vertebrate. 



II. " Method of measuring the Resistance of a Conductor or of a 

 Battery, or of a Telegraph-Line influenced by unknown Earth- 

 currents, from a single Deflection of a Galvanometer of unknown 

 Resistance." By Henry Mance, Superintendent Mekran Coast 

 and Persian Gulf Telegraph Department, Kurrachee. Commu- 

 nicated by Sir Wm. Thomson. Received January 12, 1871. 



The resistance of each part of a circuit, such as that shown in fig. 1, 

 being known, the influence exercised by the shunt A B, as well as the 



