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In the course of an investigation into the possibility of improving the digestibility of straws by biological pretreatment with cultures from the sheep's rumen, it became evident that no very clear picture was available, from previous studies, of the process of ruminant digestion of untreated straws. The literature contains numerous references to digestibility trials on a variety of straws. The results are, for the most part, presented in terms of such fractions as crude

1. The concentration of volatile acids in the blood draining the rumen is considerably higher than that of peripheral blood in which little volatile acid, if any, is present. 2. Volatile acid in significant amount is present in blood draining the omasum and the caecum but is absent from blood draining the abomasum and small intestine. 3. Volatile acid is also found in significant concentration in blood draining the caecum or colon of the horse, pig and rabbit. 4. Distillation curves indicate that acetic is the predominant acid present in both the rumen contents and in the blood draining the rumen, but the proportion of acids of higher molecular weight than acetic is greater in the rumen contents than in the blood. The same type of distillation curve is found for material taken from the caecum of the sheep and the large intestine of the horse and pig. 5. The rate of absorption of the sodium salts of acetic, propionic and butyric acids, as judged by the concentrations found in the blood leaving the rumen, appears to be in the order of their molecular weights; acetate is rapidly absorbed, propionate less so, while butyrate is slowly absorbed. 6. The quantity of volatile acid calculated as acetic acid absorbed in an hour from the reticulum and rumen together is estimated to vary from 1 to 5 g., but this value must be regarded as minimal.

Summary 1. The diet of ruminants is peculiar in that it contains large quantities of cellulose and other carbohydrates which are indigestible except by the aid of bacteria; it is probable that the numerous protozoa of the rumen do not play a vital part in digestion. 2. Food passes to the fermentation chamber formed by the reticulum and rumen where it is kept in continual motion by rhythmic contractions of this organ. Regurgitation is caused by an inspiratory effort with a closed glottis; the bolus after being remasticated returns to the fermentation chamber. 3. Passage of ingesta from the rumen is slow and spread out over a period of days. Digestion is more efficient in the ruminant than in the horse and pig owing to the greater time spent by the food in its alimentary tract rather than owing to the act of remastication. 4. The voluminous secretion of saliva is responsible for the maintenance of a moisture content and reaction of the ingesta of the rumen suitable for the bacterial fermentation of carbohydrate. The conditions are anaerobic, since carbon dioxide and methane are the chief gases present, oxygen being found only in traces, if at all. 5. The breakdown of cellulose and other carbohydrates is accomplished mainly by bacteria, though plant enzymes may play a minor part. The products of this fermentation are largely carbon dioxide, methane and the volatile fatty acids, though it has been suggested that a starch‐like polysaccharide is synthesized. The degree of digestion of cellulose is affected by the other constituents of the diet. 6. The rate and degree of digestion observed by the use of an indigestible constituent of the food as an index show that the total digestion and in particular that of cellulose in the rumen is considerable within 14 hr.; this is supported by measurements of the rate of evolution of methane. 7. Absorption of the volatile fatty acids and of certain drugs has been proved to occur from the rumen. 8. Fermentation is not confined to the rumen; it also occurs in the caecum of the ruminant. The authors are greatly indebted to Sir Joseph Barcroft, F.R.S., for his constructive criticism and help in the preparation of the manuscript.

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