This statement, made in 1905, was the first clear formulation of Langley's receptor concept. He was very quick to generalize it, suggesting that it could be applied to explain also the action of other alkaloids, such as pilocarpine, atropine and strychnine, and of hormones, such as adrenalin, secretin, thyroidin and the sex hormones. And he wrote in general terms:So we may suppose that in all cells two constituents at least are to be distinguished, a chief substance, which is concerned with the chief function of the cell as contraction and secretion, and receptive substances which are acted upon by chemical bodies and in certain cases by nervous stimuli. The receptive substance affects or is capable of affecting the metabolism of the chief substance[16].