358 SECRETION. 



Aside from the mechanical functions of mucus, it has been shown that this fluid, in 

 connection with the epithelial covering of the mucous membranes, is capable of prevent- 

 ing the absorption of certain substances. It is well known, for example, that venoms may 

 be applied with impunity to certain mucous surfaces, while they produce poisonous 

 effects if introduced into the circulation. These agents are not neutralized by the secre- 

 tions of the parts, for they will produce their characteristic effects upon the system when 

 removed from the mucous surfaces and introduced into the circulation ; and it is reason- 

 able to suppose that the mucous membranes are capable of resisting their absorption. 

 This fact is proven by the following interesting experiment, detailed by Robin : 



Let an endosmometer be constructed, using a fresh mucous membrane, on the surface 

 of which the epithelium and layer of mucus remain intact, and in the interior of the 

 apparatus, place a saccharine solution, and let the membrane be exposed to a solution con- 

 taining some venomous fluid. The liquid will mount in the interior of the apparatus, but 

 the poison will not penetrate the membrane. If the mucus and epithelium be now 

 removed with the finger-nail from even a small portion of the membrane, the poison will 

 immediately pass through that part of the membrane, and an animal may be killed with 

 the fluid which now penetrates into the interior of the endosmometer. 



These facts show that mucus is an important secretion. It not only has a useful me- 

 chanical function, but it is in all probability closely connected with some of the phenomena 

 of elective absorption which are so often observed, particularly in the alimentary canal. 



Sebaceous Fluids. 



The general cutaneous surface is constantly lubricated by a small quantity of a pecul- 

 iar, oily secretion, called sebum, or sebaceous matter. This secretion is somewhat modi- 

 fied in certain situations, and an analogous fluid is produced by special glands opening 

 into the external meatus of the ear. Another fluid, very much like the ordinary seba- 

 ceous matter, is smeared upon the edges of the eyelids. These secretions, called respec- 

 tively cerumen and Meibomian fluid, resemble the secretion of the ordinary sebaceous 

 glands sufficiently to be classed with it. 



Physiological Anatomy of the Sebaceous, Ceruminous, and Meibomian Glands. The 

 true sebaceous glands are found in all parts of the body that are provided with hair ; and, 

 as nearly every part of the general surface presents either the long, the short, or the 

 downy hairs, these glands are very generally distributed. They exist, indeed, in greater 

 or less numbers in all parts of the skin, except the palms of the hands and the soles of 

 the feet. In the labia minora in the female, and in portions of the prepuce and glans 

 penis of the male, parts not provided with hair, small, racemose sebaceous glands are 

 found, which produce secretions differing somewhat from that formed by the ordinary 

 glands. The glands in the areola of the nipple in the female are very large and are con- 

 nected with small, downy hairs. 



Nearly all of the sebaceous glands are either simple racemose glands, that is, present- 

 ing a number of follicles connected with a single excretory duct, or compound race- 

 mose glands presenting several ducts, with their follicles, opening by a common tube. 

 Although there is this variation in the size and arrangement of the glands of the general 

 surface, they secrete essentially the same fluid, and their anatomical differences consist 

 simply in a multiplication of follicles. 



The differences in the size of the sebaceous glands bear a certain relation to the size 

 of the hairs with which they are connected ; and, as a rule, the largest glands are con- 

 nected with the small, downy hairs. These distinctions in size are so marked, that the 

 glands may be divided into two classes; viz., those connected with the long hairs of the 

 head, face, chest, axilla, and genital organs, and the coarse, short hairs, and those con- 

 nected with the fine, downy hairs. 



