The Lever ^ the Bit and Curh^ etc. 175 



act on the bars of the lower jaw should come into con- 

 tact with them, and with them alone, and in the degree 

 required ; and that, on the other hand, that portion des- 

 tined to act on the tongue should be of exactly the 

 proper dimensions and form. Of course there is a great 

 difference in this respect between smooth mouthpieces 

 and such as have a port ; in fact, it is only as regards 

 the latter that the dimensions are important. Where, 

 then, a port exists, its width should be exactly that of the 

 tongue-channel, as otherwise it would either intrench on 

 the space allotted to that portion of the mouthpiece 

 required for the bars, and produce the inconveniences 

 alluded to above ; or, if narrower, it would fail to an- 

 swer the purpose for which it is intended, namely, to 

 admit the tongue.* The width of the port 7mist be^ 

 therefore^ exactly that of the tongue-channel — and this 

 is the second grand rule as regards the mouthpiece. 

 Now it has been already shown that the width of the 

 tongue-channel is very constantly three-fourths of the 

 height of the bars, which being equally constantly 1.8 

 inches, we have i^ inches for the maximum width of the 

 port, even in case where the total width of the mouth, 

 and consequently of the mouthpiece, amounts to 4I and 

 5-j^ English inches : for pony and hack bits about i inch 

 will suffice ; whereas the common practice of the bit- 

 makers seems to be to make it one-third of the total 

 width in all cases. 



For the height of the port, of course, no rule can be 

 given, this being precisely the most variable dimension 

 of all, and depending altogether, so far as the interior 

 conformation of the mouth is concerned, on the relative 

 thickness of the tongue and sensitiveness of the bars ; 

 and further, as we have already shown, on the tempera- 



* The Germans call the port of a bit the " tongue-freedom " — Zun- 

 genfreiheit — which expresses exactly the purpose for which it is in- 

 tended. 



