THE ACTION OF THE MAMMARY GLAND. 233 



substances dissolve the membrane, and then the ether takes up the fat 

 and forms a dimly-clear solution. We may therefore conclude that the 

 substance we designate as casein consists of two ingredients, the protein 

 compound, which exists in a state of solution in milk, and also that 

 which forms the membrane of the fat corpuscles. 



Many of the remarks just made respecting the origin of casein are ap- 

 plicable to the saccharine constituent of the milk, the origin origin of the 

 of which is not to be attributed so much to the food directly sugar of milk. 

 as to the system ; for, in starvation, the sugar, like the casein, still con- 

 tinues to form to nearly the normal amount. I think it is probable that 

 its production is due to the liver, and is, in reality, nothing more than an 

 indication of the continued action of that gland, one of the prime func- 

 tions of which is the generation of saccharine compounds. 



From the data now before us respecting the origin of the different con- 

 stituents of the milk, the casein, the butter, the sugar, and T ' he mammary 

 the salts, we are able to come to a definite conclusion re- gland acts by 

 garding the physiological action of the mammary gland. I 

 have entered on this long disquisition from the important bearing which 

 the decision we arrive at has upon the whole theory of secretion ; for if 

 there be a gland in the body in which we should expect to find proofs 

 of formative power, through the agency of cell life or otherwise, in giving 

 rise to products that did not pre-exist in the blood, it is certainly the 

 mammary. But now, as it appears that all the constituents which its 

 secretion contains are found in the blood, we can scarcely suppose that 

 the gland itself does more than merely strain them out ; of course, in com- 

 mon with all such structures, it possesses what might aptly be termed 

 an elective filtrating power ; thus it permits the exudation of the iodide 

 of potassium from the blood, but refuses a passage to the ferrocyanide. 

 And, finally, the conclusion to which, we thus come recalls the remark 

 heretofore made, that the more thoroughly we study the secretions deliv- 

 ered by the various glands, and the more perfectly we identify the sources 

 from which their constituent ingredients have been derived, the more we 

 should be disposed to impute glandular action to the physical process of 

 elective filtration, and the less to the agency of cell life. 



OF THE SKIN. 



The skin is composed of two layers, the epidermis or cuticle, and the 

 derma or cutis. It contains tw9 systems of glands, one for the removal 

 of water, and another for that of oily substances. It also presents sub- 

 sidiary parts or appendages, such as the nails and hair. 



The epidermis, which is the exterior portion of the skin, originates from 

 the cutis. It has a different thickness in different parts; The epidermis : 

 the contrast, in this respect, being very well shown upon the its structure. 



