THE SKIN AND ITS APPENDAGES. 419 



immediately on exposure to the air it solidifies to a white, tallow-like 

 material. Its reaction is alkaline. 



The most important constituents of the sebum, as also of the 

 related secretion of the uropygian gland of birds, are the compound 

 cholesterins (see page 80). Their presence has been demonstrated 

 on the feathers and bills of birds, in the wool of sheep, in the hair of 

 mammals, on the spikes of porcupines, etc. ; and as these bodies are 

 remarkably resistant to the influence of putrefactive organisms, it 

 has been supposed that their presence on the skin and its append- 

 ages serves the purposes of protecting the exposed portions of the 

 body against bacterial invasion. 



In addition to substances of this order, the sebum contains a vari- 

 able amount of fats, fatty acids, soaps, lecithins, mineral salts, and 

 at least two albumins, of which one is commonly regarded as casein. 



Closely related to the common sebum of the sebaceous glands of 

 the skin proper is the secretion of the preputial gland the so-called 

 smegma prseputii and the cerumen of the ceruminous glands of the 

 external ear. In addition to the common constituents of the sebum, 

 the smegma also contains certain components of the urine and their 

 decomposition-products, such as ammonium soaps, and in the horse 

 hippuric acid, benzoic acid, and oxalate of lime. Its peculiar odor 

 in man is no doubt due to the presence of certain fatty acids. In 

 the secretion of the beaver the castoreum of the shops this is 

 thought to be referable to a phenol-like body, while in the corre- 

 sponding product of the musk-deer (muse) a volatile base is, accord- 

 ing to Wohler, the active odorous principle. 



The cerumen differs from the common sebum in containing a 

 very considerable proportion of potassium soaps. In addition, we 

 also meet with a peculiar yellow pigment, which has an exceedingly 

 bitter taste ; but of the composition of this nothing is known. 



In the skin glands of certain amphibia, finally, and notably the 

 toad and the salamander, poisonous substances have been found 

 which in their physiological effect closely resemble the action of 

 digitalis and strychnin. They have been termed bufidin and sala- 

 mandrin, respectively. In the secretion of the toad, moreover, 

 methyl-carbylamin and isocyan-acetic acid have been found, of which 

 the former is especially toxic. 



