The observations of the craniologist are continually liable to error. The irregular thickness of the skull constitutes a great difficulty in the way of exact observations. By great expertness and accuracy of observation, he may overcome this difficulty in a great degree, but whenever the brain is subject to any remarkable influence, increasing or diminishing the activity and size of particular organs, the external form fails to indicate the internal condition, because it can change but slightly, and with slowness, after the skull is fully developed and ossified. Were the skull composed of more pliable materials, cranioscopy would be more accurate in its facts, but while it preserves a uniform exterior, the interior often undergoes remarkable changes. Convolutions that are frequently called into action become better supplied with arterial blood, expand and grow, while the adjacent portion of the inner plate of the skull becomes absorbed, and presents a remarkable indentation. Convolutions that are seldom in action shrink in size, and the adjacent bone grows in upon them. Thus the skull becomes thinner at the site of every active organ, and thicker over every convolution that is inactive. The translucency or opacity of the different parts of the skull, when a light is placed in its interior, generally indicates the active and inactive organs. Hence, many skulls of fine exterior reveal, upon interior examination, a degenerate character. Criminal heads generally present remarkable opacity and thickness in the region of the moral organs, with distinct digital impressions from the convolutions of the lower organs. Thus all craniological observations are liable to inaccuracy, even as regards development, and much more in regard to functional power. The activity, power and predominance of an organ may be essentially changed, without making any perceptible impression upon the interior of the skull, for an indefinite period. Changes in excitement and circulation, that revolutionize the character, may leave but a slight impression upon the interior, and none upon the exterior of the cranium. The external configuration of the skull is therefore not a true criterion of character when the influences of education, society, food, drink and disease have greatly changed the natural bias, although reliable in a strictly normal condition of brain and cranium.