Chap. II.] 
COURSE OF THE SAP. 
75 
piths are so far distinct from the wood, that 
what are absurdly called, and commonly be¬ 
lieved to be, shakes arise in them. All the 
shakes which I have observed show on each side 
of the tree, from which I imagine they must be 
the result of disease in the original medullary 
rays of the seedling: for one can scarcely sup¬ 
pose sympathy between two opposite new me¬ 
dullary rays which have no junction with each 
other; nor would one have anticipated this in 
two opposite original rays. That the shakes 
pass through the bark shows, I think, that the 
rays of the wood communicate with those of the 
bark. These shakes often rise to a great height, 
and are never cured. A diseased concentric pith 
is called a cup-shake . 
The concentric pfth, or cup-shake, may be 
called finite ; the medullary ray shake, infinite, 
by comparison : that is, a cup-shake is conical, 
and cannot extend above the cone of the year’s 
growth in which it is generated. All the timber, 
therefore, which is above that cone, or outside it, 
is sound. The medullary ray shake may be con¬ 
tinually prolonged upward, downward, and out¬ 
ward and inward, with the growth of the tree. 
The new medullary rays proceeding from the 
new rings of pith may be easily seen in oaks: 
