166 THE MICROSCOPE. 
in developing the shape, proportions and structure of the teeth. 
Everyone knows that exercise is essential to a healthy or proper de- 
velopment of the muscles. No one doubts but that the strength of 
the arm of the blacksmith is due, to a large extent, to the exercise it 
receives. Few, however, stop to consider that upon a similar in- 
fluence depends the character of a tissue like the teeth. EH. D. Cope 
has pointed out the effect of ‘impact and strains on the feet of 
mammalia,” in a paper read before the National Academy of Science. 
He says: ‘ There is every reason to believe that shocks, if not too 
severe, encourage growth in the direction applied.” 
I fully indorse what he says there, and will add that such in- 
fluence determines the character of the tissue in the parts so 
influenced. Apply this to the teeth. Note the size and strength of 
the incisors of the rodentia; the canine of carnivorous animals, also 
the rudimentary nature of their incisors. Now there can be no doubt 
but use is the cause of the excessive development in the first case 
and want of use the cause of the rudimentary condition of the latter. 
Now how does this apply in a pathological sense? It seems to me 
this will show the necessity of perfect articulation. 
A tooth that is not opposed by another tooth must always be 
deficient in structure, and will always lack the proper quantity of 
earthy matter. In other words, it is soft and soon becomes affected 
with caries. It is a very unobserving dentist who has not noted that 
perfect articulation is essential to a healthy condition of the teeth. 
The reason is clear. The influence of “‘ impact” has developed the 
structure and caused a proper deposit of earthy material where the 
articulation is perfect, so as to harden the tissues and thus increase 
their power of resistance to the action of acids. 
This being the case, any abnormal condition that would prevent 
a perfect articulation is to be considered a predisposing cause of 
caries. 
The active or immediate cause of caries is nothing more or less 
than the various acids either taken into the mouth as food, or with 
the food, or generated there. Most acids that find their way into 
the mouth are injurious to the teeth, such as fruit acids, vinegar, 
lactic acid, and acids may be generated in the mouth that will act 
upon the teeth; but whatever influence fungi may have in this, if any, 
would be indirect and comes from the decomposing food material, 
and not from the tissue of the teeth. 
The saliva, which is generally alkaline, plays a part in softening 
and loosening the remaining organic material (which is, of course, 
dead after the acid has done its work), exposing fresh surfaces to the 
