CHAPTER XVIII. 



THE SKIN. 



BY A. R ROBINSON, M.D., 



Lecturer on Normal Histology in the Bellevue Hospital Medical College, New York. 



General plan of arrangement. The integumentum commu- 

 ne, or skin, forms the external covering of the body, which 

 it mechanically protects, and at the same time is endowed 

 with certain physiological functions. The surface of the skin 

 in some parts of the body is smooth and soft ; in others it 

 is more or less uneven and rough. This latter condition 

 depends upon the presence of pores, hairs, furrows, and 

 ridges. The pores correspond to the surface openings of the 

 hair-follicles, sebaceous and sweat-glands. The hairs vary 

 in amount of development according to their situation. In 

 the so-called hairy regions they are largest ; other parts are 

 provided only with a soft down (lanugo hairs). There are 

 no hairs on the palms of the hands and soles of the feet, the 

 dorsal surfaces of the terminal phalanges of the fingers and toes, 

 the glans penis, and inner surface of the prepuce. The fur- 

 rows are either long and deep, or short and superficial. The 

 former are chiefly found in the flexures of the joints, and cor- 

 respond to the folds in the derma produced by movements of 

 the joint. The latter run between the papillary elevations, 

 and, by crossing each other, divide the surface into a number 

 of polygonal or lozenge- shaped fields. This division is well- 

 marked on the backs of the hands. These superficial furrows 

 are more developed on the extensor than on the flexor surfaces 

 of the extremities, and in the lumbar region more than on the 

 anterior surface of the abdomen. Their direction is dependent 

 on the degree of the tension of the skin. The ridges correspond 

 to the papillae, and are most developed on the palmar surfaces 

 of the last digital phalanges. The color of the skin varies in 



