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54 diseases. But however this may be, the perfect mobility of the pigment granules seems to me a special property which they possess as constituents of the healthy living body; in other words, to use once more the expression which in the present state of our knowledge is indispensable, a vital property.

If this be so, we understand what would otherwise be very unintelligible, viz., that when the pigment cells have their functions temporarily suspended by a noxious agent, the granules do not become diffused as they do when simply withdrawn from the influence of the nervous centres, but remain exactly as they were before the irritant was applied, whether fully concentrated, completely diffused, or in any intermediate state. If we suppose that the pigment granules, like the blood-corpuscles, acquire under irritation a tendency to mutual aggregation which they do not possess in health, it follows, as a matter of course, that when vital energy is suspended by the noxious agency, they will adhere together and retain their relative positions.

After being appointed to the Chair of Surgery in the University of Glasgow, I became one of the surgeons to the Royal Infirmary of that city. Here I had too ample opportunity for studying Hospital diseases, of which the most fearful was pyaemia. About this time I saw the opinion expressed by a high authority in pathology that the pus in a pyæmic vein was probably an accumulation of leucocytes. Facts such as those which I mentioned as having