320 SECRETION. 



been dried, be exposed to water, it assumes the viscid consistence peculiar to 

 mucine. This property serves to distinguish it from albumin and other or- 

 ganic nitrogenized matters. 



General Uses of Mucus. The smooth, viscid and adhesive character of 

 mucus, forming, as this fluid does, a coating for the mucous membranes, 

 serves to protect these parts, enables their surfaces to move freely one upon 

 the other, and modifies to a certain extent the process of absorption. Aside 

 from these mechanical uses, it has been shown that mucus, 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 secretions 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 reasonable 

 to suppose that the mucous membranes are capable of resisting their absorp- 

 tion. This fact is illustrated by the following experiment : 



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 mem- 

 brane be exposed to a solution containing some venomous fluid. The liquid 

 will mount in the interior of the apparatus, but the poison will not pene- 

 trate 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 (Robin). 



These facts show that mucus is an important secretion. It not only has 

 a useful mechanical office, 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. 



Physiological Anatomy of the Sebaceous, Ceruminous and Meibomian 

 Glands. The true sebaceous glands are found in all parts of the skin that 

 are provided with hair ; and as nearly every part of the general surface pre- 

 sents 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 connected with small, downy hairs. 



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

 is, presenting a number of follicles connected with a single excretory duct, or 

 compound racemose glands, presenting several ducts, with their follicles, 

 opening by a common tube. Although there is this variation in the size and 



