The ferment pepsin can exert its _maximum_ action only in the presence of free hydrochloric acid. There must be sufficient HCl to combine with all of the proteid matter present, and the products of proteolysis as fast as they are formed, if digestion is to be rapid and attended with the formation of a large proportion of the final products of proteolysis. It is under such conditions that our study of pepsin-proteolysis is usually conducted. Further, it is to be remembered that our knowledge of the products of such proteolytic action depends almost entirely upon data accumulated by artificial digestive experiments. In no other way can we be absolutely certain of the conditions under which the proteolysis is accomplished, for it is a significant fact, perhaps plainly evident from what has already been said in the preceding lecture, that the character of the products resulting from ordinary proteolysis is dependent in great part upon the attendant circumstances. Thus, with a relatively small amount of acid, and perhaps also of pepsin, the initial products of proteolysis are especially prominent, while with an abundance of both pepsin and free acid, coupled with long-continued action, the final products predominate. Between these two extremes there are many possible variations, as was, I think, made clear in the previous lecture. At the same time, it is to be noticed that these differences are mainly differences in the _proportion_ of the several products, rather than in the nature of the resultant bodies. In a general way, the products of pepsin-proteolysis may be divided into three main groups, viz., bodies precipitated by neutralization and represented mainly by the so-called syntonin or acid-albumin; bodies precipitated by saturation of the neutralized fluid with ammonium sulphate and represented by proteoses; bodies non-precipitable by saturation with ammonium sulphate and represented by amphopeptones. The relationship of the individual products may be clearly seen from the following scheme, arranged after the plan suggested by Neumeister. NATIVE PROTEID. | | Syntonin. ╱ ╲ ╱ ╲ ╱ ╲ Protoproteose. Heteroproteose. | (dysproteose). | | Deuteroproteose. Deuteroproteose. | | Peptone. Peptone.