factor of this irritability _Glisson_ attributes to the “_perceptio_,” which he distinguishes as a “_perceptio naturalis_, _sensitiva_ and _animalis_.” The want of clearness produced here by _Glisson’s_ artificial distinctions and mode of expression is in part removed if we endeavor to transfer his meaning into our present methods of thought. This distinction would then simply point out the different means by which the stimuli can reach the irritable structures. The “_Perceptio naturalis_” is that which today we should call “direct response” to stimulation, that is, the excitation of the fiber by artificial stimuli applied directly to the tissue. _Glisson_ shows here, that the intestines and muscles in the body immediately after death and even when removed from the body can be stimulated to movement by means of corrosive fluids or cold. The “_Perceptio sensitiva_” is, according to _Glisson_, the excitation of the fibers by external stimuli which act on the intact body as a whole by way of the sensory nerves. The “_Perceptio ab appetitu animali regulata_” finally is the excitation by inner stimuli proceeding from the brain. The _Perceptio naturalis_ is possessed by all parts of the body, even the fluids, the bones and the fat. All of them are irritable. But a “vitale” and a special “animal” irritability they do not possess to a perceptible degree. These forms of irritability belong only to the special parts of the body. Here, however, the distinctions made by _Glisson_, are quite vague and contradictory. In his “_Tractatus de ventriculo et intestinis_” _Glisson_ sharply distinguishes the “_sensatio_” from the “_perceptio_.” The perceptio in itself is not a sensation, for although individual organs of the body are irritable, as they all possess a “perceptio,” they are not in themselves sensitive. The “_sensatio_,” the sensation, only arises when the external “_perceptio_” of the individual organs combine through the nerves with the internal “perceptio” of the brain. “_Nisi enim percepto externa ab interna simul percipiatur, non est cognitio sensitiva completa._” Sensitivity is, therefore, a special faculty, that is only based upon irritability. [1] _Franciscus Glissonius_: “Tractatus de natura substantiæ energetica seu de vita natura ejusque tribus primis facultatibus perceptiva, appetitiva, motiva,” etc. Londini M D C L XXII. [2] _Franciscus Glissonius_: “Tractatus de ventriculo et intestinis cui præmittitur alius de partibus continentibus in genere et in specie de iis abdominis.” Amstelodami M D C L XXVII.