92 SECRETION. 



the microscope, the globules become deformed, and some of 

 them show a tendency to run together ; an appearance which 

 is supposed by Henle, who was the first to study closely the 

 action of acetic acid upon the milk-globules, to indicate the 

 existence of a membrane. 1 This deduction, however, is not 

 justifiable. Acetic acid readily coagulates the caseine, a 

 principle which is most efficient in maintaining the fat in its 

 peculiar condition. The coagulating caseine then presses 

 upon the globules, and produces, in this way, all the changes 

 in form that have been observed. 



Most of the other arguments in favor of the existence of 

 a membrane have no support in direct observation, and con- 

 sequently do not demand special consideration ; while all the 

 facts which we have been able to find relating to this sub- 

 ject go to show that the fatty matters in the milk are in the 

 condition of a simple emulsion. The precise condition, 

 however, of the fluid immediately surrounding the globules 

 is not fully understood. Certain of the constituents of fluids 

 capable of forming emulsive mixtures with liquid fats may 

 form a coating of excessive tenuity immediately around 

 the globules, but they never constitute distinct membranes 

 ^capable of resisting the action of solvents upon the fats ; and, 

 in the case of the milk, they do not prevent the mechanical 

 union of the globules into masses, as occurs in the process 

 of churning. 



Milk-globules less than -g-^Vo f an mcn ^ n diameter pre- 

 sent under the microscope that peculiar oscillating motion 

 known as the Brownian movement. This is arrested on the 

 addition of acetic acid, by coagulation of the caseine. 



From these facts, it is evident that the milk-globules are 

 composed simply of fat in the condition of a fine emulsion. 

 They are not true anatomical elements, originating by a 

 process of genesis in a blastema, undergoing physiological 

 decay, and capable of self-regeneration from materials fur- 

 nished by the menstruum in which they are suspended, like 



1 HENLE, Traite d> anatomic generate, Paris, 1843, tome ii., p. 521. 



