40 THE SKIN, ITS APPENDAGES AND ITS FUNCTION. 



ing blood-vessels, lymphatics, and nerves, as well as un- 

 striped muscle fibers where hair is present. 



At the apertures .of the body the skin stops and is 

 replaced by mucoiis membrane, an integument of greater 

 delicacy but which consists fundamentally of the same 

 two layers, a superficial, bloodless epithelium and a 

 deep fibrous derma. It is continuous with the skin, 

 but is much redder and more sensitive and bleeds 

 more easily. The passages and cavities that it lines, 

 unlike those lined by serous membranes, communicate 

 with the exterior of the body and are for that reason 

 protected against contact with foreign substances by 

 mucus, which is thicker and more sticky than the 

 lymph that moistens the endothelium found on serous 

 surfaces. Mucous membrane is found in the alimentary 

 canal, the respiratory tract, and the genito-urinary tract. 

 In cavities, like the stomach and intestines, which are 

 subject to variations in capacity, it is thrown into folds 

 or rugae. The mucus is secreted by small glands in the 

 membrane. 



Appendages of the Skin. The skin has various ap- 

 pendages. On the dorsal surface of the last phalanges 

 of the fingers and toes are flattened and horny modi- 

 fications of epithelium, the nails. They have a root 

 embedded in a groove of skin by which they grow in 

 length and a vascular matrix of derma beneath them 

 which gives growth in thickness. To their growth 

 in length there seems to be no limit. 



The hairs also, which occur all over the body, ex- 

 cept on the palms of the hands and the soles of the 

 feet, are a modification of the epithelium. Each hair 

 has a bulbous root springing from an involution in the 

 epidermis and derma called the hair follicle, into which 

 one or two sebaceous glands empty. It is raised by 

 involuntary muscle fibers and grows by constant ad- 

 ditions to the surface by which it is attached. This 

 growth seems, however, to be limited, and when its 

 term is reached the hair falls out and is replaced by 



