105 



LESSON XXIII. 



THE SKIN. 



1. SECTIONS of skin from the palmar surface of the fingers. The sections 

 are to be made vertical to the surface, and should extend down as far as the 

 subcutaneous tissue. They may be stained with logwood or picro-carmine 

 and mounted in Canada balsam. In these sections notice the layers of the 

 epidermis and their different behaviour to the staining fluid. Notice also the 

 papillae projecting from the corium into the epidermis, and look for tactile 

 corpuscles within them. In very thin parts of the sections the fine inter- 

 cellular channels in the deeper parts of the epithelium (see Lesson VI. p. 22) 

 may be seen with a high power. The convoluted tubes of the sweat-glands 

 will be seen here and there in the deeper parts of the corium, and in thick 

 sections the corkscrew-like channels by which the sweat is conducted through 

 the epidermis may also be observed. Make a sketch showing the general 

 structure under a low power, and other sketches to exhibit the most important 

 details under a high power. Measure the thickness of the epidermis and the 

 length of the papillae. 



2. Sections of the skin of the scalp, vertical to the surface and parallel to 

 the slope of the hair-follicles, and others parallel to the surface, and therefore 

 across the hair-follicles. Stain and mount in the same way as in the last 

 preparation. Examine also the structure of the hairs. 



In these preparations the details of structure of the hairs and hair-follicles 

 together with the sebaceous glands and the little muscles of the hair-follicles 

 are to be made out. 



3. Vertical sections across the nail and nail-bed, cut with a strong 

 scalpel or razor. The sections are stained with haematoxylin or picro-carmine. 

 Notice the ridges (not papillae) of the corium projecting into the epidermis. 

 Observe also the distinction of the epidermis into Malpighian layer and nail 

 proper. 



4. Mount in Canada balsam a section from a portion of skin of which the 

 blood-vessels have been injected, and notice the distribution of the capillaries 

 to the sweat-glands, to the hair-follicles, and to the papillary surface of the 

 corium. 



The skin is composed of two parts, epidermis and cutis vera. 



The epidermis, or scarf skin, is a stratified epithelium (fig. 129). 

 It is composed of a number of layers of cells, the deeper of which are 

 soft and protoplasmic, and form the rete mucosum of Malpighi, whilst 

 the superficial layers are hard and horny ; this horny portion some- 

 times constituting the greater part of the thickness of the epidermis. 

 The deepest cells of the rete mucosum, which are set on the surface of 

 the cutis vera, are columnar (fig. 129, c\ in shape. In the coloured 



