In a previous paper the point has been brought out that the influence of the gall bladder upon the bile differs entirely from that of the ducts, the one organ acting to concentrate the secretion markedly and the other to dilute it slightly. The question arises, in species lacking a gall bladder, whether the concentrating function of this organ will be found lodged in the ducts. To test the point, observations have been made upon the mouse and rat, since these animals though so nearly related have, the mouse, a gall bladder and the rat, none.
The normal bile was first studied. Both animals were found to secrete larger quantities than do cats and dogs, but less than the guinea pig and rabbit. Methods were worked out for the quantitation of the pigment which was used as the index to changes in concentration.
Bladder bile of the mouse was regularly found to be more concentrated than that collected from the common duct of the same animal. The bile collecting during stasis regularly showed a great increase in pigment content, such as in other species is brought about by the action of the gall bladder. In the rat, on the other hand, stasis bile never became more concentrated in pigment than the normal.
The gall bladder, then, is not only absent from the rat in form, but in one at least of its important functions. That its other obvious function—that of a reservoir—cannot be assumed in the rat by the ducts would seem to be indicated, not only by the small size of these channels, but by the recent observation of Mann that the tonus of the sphincter of Oddi is almost negligible in the rat, in contradistinction to animals which possess a gall bladder.
It is an interesting fact that the bile of the rat, which as has been said, undergoes no condensation of bulk after leaving the liver, contains on the average eight times as much pigment as does the liver bile of the mouse which is submitted to concentration. Whether it is correspondingly strong in substances useful for digestion, and therefore ab initio requires no concentration, is a matter upon which little can be said at present. However, in this connection the fact that the bulk of bile secreted per gram of liver weight is identical in both animals may be significant. Although this output is the same, the mouse liver when compared with the body weight (1 to 14.6) is relatively larger than that of the rat (1 to 21.7), so that the mouse secretes somewhat more bile per 100 gm. of body weight. This bile as it comes from the liver is but one-eighth as strong at least in pigment as rat bile, but the concentrating activity of the gall bladder is so great that the products yielded to the intestine may become not dissimilar.