The influence of the moon on the mind was taken quite seriously. Rush seems to have been somewhat in doubt on this subject and suggested the probability of there being a kind of sixth sense involved—a perception of the state of the air, and of light and darkness, as Pritchard expressed it, to which we are insensible in health. It was thought that the full moon, by rarefying the air, increased the amount of light, thus affecting the mind. Dr. Rush noted that during an eclipse of the sun in 1806 "there was a sudden and total silence in all the cells of the hospital." He expressed the opinion in his "Medical Inquiries and Observations" in 1812 that there are few cases in which the insane feel the influence of the moon and that the excitement resulting in such cases is to be attributed to the resulting increase of light. It is interesting to note that von Feuchtersleben, an eminent German writer, in 1845 was unwilling to go on record as stating positively that the moon was not a factor in the causation of insanity. Esquirol, in his "Maladies Mentales," in 1838, branded this belief as a superstition, but admitted that there were certain facts which could not be overlooked. "It is true that the insane are more agitated at the full moon as they are also at the dawn of day; but is it not the bright light of the moon that excites them, as that of the day every morning? Nevertheless, an opinion which has existed for ages—which has spread over all lands, and which is consecrated by popular language—demands the most careful attention of observers." Dr. Allen of the York Lunatic Asylum was very firmly of the opinion that the moon had a decided influence on the time of death in mental diseases. This question was given very serious consideration by various writers as late as 1856. In the meanwhile efforts were being made to ascertain the cause of mental disease by means of pathological researches. Morgagni,[59] one of the earlier investigators, came to the conclusion that the more common lesions were in the pineal gland, although he found some induration of the brain and various other well-defined changes. Arnold (1782) thought that insanity was due to an increased density of the cerebral substance, particularly, according to Tuke, "of those parts of the brain by means of which the soul is connected with the body." Pinel finally concluded that pathology had practically nothing to do with the problem and Esquirol in 1838 wrote very discouragingly on the subject. Early contributions of considerable importance were made, however, by Foville, Bayle, Greding, Calmeil, Guislain, Parchappe and others. These were confined almost entirely to a study of gross or macroscopic lesions of the brain. Griesinger in 1845 reviewed the pathological changes in the nervous system quite thoroughly as far as they were known at that time. It must be admitted that the greater part of our knowledge of the pathology of mental diseases was acquired at a much later date.