This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
As indicated by the name, heartshake is a defect which shows itself at the heart of the tree in the center of the trunk. The appearance of a cross section of a log affected by heartshake is shown in Fig. 4. There is first a small cavity at the center caused by decay, and flaws or cracks extend from this cavity outward toward the bark. The heartshake is most often found in those trees which are old, rather than in young, vigorous saplings: it is especially to be feared in hemlock timber.