THE NECK, THE HEAD, ETC. 135 



as it would be to put the lock of a gun somewhere near 

 the vent an inch or two higher or lower does not 

 matter. As far as the bit itself is concerned, half- 

 an-inch, or even a full inch, higher or lower, would 

 not make much difference in its action ; but, as 

 we shall presently see, the curb has one fixed 

 position in the chin-groove, and this governs the whole 

 arrangement. 



There are three dimensions of the interior of the 

 horse's mouth which must be accurately ascertained 

 before attempting to fit him with a proper bit, in 

 addition to certain details connected with the tongue. 

 The first, and a very important one, is the transversal 

 width of the mouth from side to side, measured at the 

 same height as the chin-groove, and including the 

 thickness of the lips that is to say, from the point 

 d, Fig. 12, to the corresponding point at the off-side." 

 This gives the width of the mouth-piece, which must be 

 made to fit exactly, as, if too narrow, the lips are subject 

 to injury and their being displaced so as to cover the 

 bars, thereby neutralising the action of the whole 

 instrument ; whereas, if too wide, it slips from side to 

 side, and displaces what is termed the port the curved 

 portion of the mouthpiece intended to moderate the 

 pressure on the tongue from its proper position : it 

 also determines the length of the curb. The second 

 dimension is the width of the channel in which the 

 tongue lies, or the distance between the two bars 

 internally, which determines how much of the mouth- 

 piece may be allowed for the port ; the remainder must 

 be reserved for the action on the bars. 



* The proper mode of ascertaining all these dimensions 

 accurately will be stated further on. 



