THE SKIN AND ITS APPENDAGES. 



71 



resembling the foetal state be thus produced. In the mucous 

 membranes of the lips, anus and vulva the superficial layer of 

 epithelium does not become cornified. 



Formation of Dermal Papillae. — About the fifth month, the 

 dermal papillae, which are grouped in lines and ridges as is well 

 seen in the palm, are formed in the following manner : 



Long, linear furrows of epidermis grow down into the dermis 

 (corium) and divide its surface into narrow ridges. These ridges 

 are subsequently subdivided into papillae. The down-growing 

 nature of the epiblastic (epidermal) cells which is here exemplified, 

 is of the greatest clinical importance. The enamel organs, we have 

 seen, arose by a species of downgrowth of the epidermis ; so do 

 hairs, sweat glands and sebaceous follicles. Prolonged pressure 

 and friction welds the corneous cells into a solid plate, such as 

 the callosities seen on the palms of manual labourers. Normal 

 desquamation is arrested ; the cells produced in the deeper layers, 

 unable to grow to the surface, grow inwards and produce corns. 

 In cancer, the epithelial cells of the skin renew their youth 

 and invade the dermis and deeper tissues. 



The papillary lines on the palms and fingers give security of 

 grasp (Hepburn). They are arranged in most variable patterns, 



A u 



e co: 

 A. The Loop Pattern. B. The Triangle Pattern. C. The Whorl Pattern. 



Fig. 54.— The more common patterns formed by the dermal papillae on the tips of 



the fingers. 



but the prevailing types in man are those arranged as loops, 

 spirals or whorls, Tig. 54. So variably are the types arranged on 

 the pulps of the digits, that probably no two people show them in 

 the same sequence counting from thumb to little finger in both 

 hands. Hence the impress of the ten finger tips has been success- 

 fully used in the identification of criminals. 



The Hairs. — Hairs begin to develop in the 5th month. 

 Morphologically a hair may be regarded as a dermal papilla, 

 which has become sunk in the subcutaneous tissue, and capped by 



