487 



occurrence, and to assume it is contrary to the fact, that we rarely 

 find any rudimental structures among the perfect ones. In the 

 most active muscles of the adult, for example, I doubt whether a 

 rudimental or developing fibre could be found ; we have sufficient 

 chemical evidence of a constant change of material in them, but no 

 evidence of an equal or parallel change of structure. And so in the 

 blood: the change of material is very rapid, but the change of 

 structures, which we may in some measure estimate by the propor- 

 tion of white or rudimental blood-cells, is probably slow*. And 

 again, in the secreting glands, excepting those of the skin and the 

 breasts, we have no evidence, and, I think, no sufficient reason to 

 believe, that in all cases the gland-cell-walls dissolve or burst in the 

 act of secretion, so as to need the entire new formation of fresh cells. 

 For we find in most of the active glands no considerable number of 

 either rudimental or degenerate cells j and the observations of 

 Ludwig and Rahn on the secretion of saliva indicate, as many other 

 facts do, that in ordinary secretion (which is the ordinary nutrition 

 of glands) the cell-contents, gradually transformed, flow out through 

 the persistent cell-walls. 



Nutritive maintenance, then, probably requires nothing more than 

 molecular substitution. Atoms even of the refuse substance may 

 be passing out, and atoms of the renewing substance passing into 

 places among the structures of a comparatively persistent frame- 

 work. Cell-walls or their analogues may be long-lived, while their 

 contents are undergoing continual mutation. Such a process of 

 molecular interchange and passage is, indeed, visible in the absorp- 

 tion of oil through the epithelial cells and villi of the intestines ; and 

 this is probably only a coarse example of the ordinary manner in 

 which cells change their contents in the nutritive processes. Changes 

 like tbese may well consist with the quickest rhythmic action. 



I would thus, then, conclude as to the most probable explanation 

 of the rhythmic action of the heart : 



1 . In the Vertebrata it is due to the time-regulated discharges of 



* The very small quantity of iron, in proportion to the quantities of the other 

 Constituents of the blood-cells, found in the excretions, is another indication of 

 the comparatively slow waste of the red blood-cells, and may suggest, besides, 

 that in nutritive maintenance there is not an equal mutation of all the compo- 

 nent substances of a structure. 



