1282 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



The aperturae cutis are holes through the skin where it joins with the mucous membrane, 

 usually without sharp line of demarcation, at the nares, the rima oris, the anus, and the external 

 urethra in the male, and at the vaginal vestibule in the female. 



Owing to the fact that the skin extends beyond the surface at the aperaturse cutis, and 

 covers the major and minor pudendal labia, and the prepuce and extends into the external 

 acoustic canal, the surface area is slightly greater than the surface of the body. 



The thickness of the skin varies in different regions of the body and also in 

 different individuals. The mean thickness is between 1 and 2 mm., the extremes 

 ranging from .3 to 4.0 mm. or more. This is exclusive of the subcutaneous tela. 

 The thickness appears to be in direct proportion to the amount of friction and 

 pressure to which the part is subjected. Thus it is thicker on the dorsal than on 

 the ventral surface of the trunk and neck, and on the flexor than on the extensor 

 surfaces of the hands and feet. Otherwise it is thicker upon the extensor than on 

 the flexor surface of the extremities. 



Fig. 1041. — Vertical Section from the Sole of the Foot of an Adult. 



Stohr.) 



X 25. (Lewis and 



Duct of a sweat gland 



Stratum corneum 

 Stratum lucidum 



Stratum granulosum 

 Stratum germinativum 



^ Corpus 



I papillare 



Epi- 

 dermis 



stratum 

 reticulare 



Corium 



Coil of a sweat gland 



Artery 



Tela subcutanea 



Fat tissue 



The thickness of the skin is least upon the tymi)auum and it is also thin upon the eyelids 

 and penis. It obtains a thickness of 3 mm. on the volar surface of hands and plantar surface of 

 the feet and gains a thickness of about 4 mm. on the cephalic part of the back and dorsal surface 

 of the neck. It is thinner in the aged than in the adult, thicker in men than in women, and 

 in the same sex is subject to much individual variation depending upon exercise, occupation, 

 etc. The vascularity of the skin also influences its thickness. 



Over most of the surface of the body the skin is elastic and so loosely attached 

 that it may be stretched to a greater or less extent. The elasticity varies in 

 different individuals. Closely associated with the elasticity is the manner of 

 attachment of the skin to underlying structures. This varies somewhat according 

 to the tissues which are covered but the great motility is due in the main to the 

 very oblique arrangement of the connective tissue and elastic fibres of the deeper 

 layers of the skin; the fixity to the more vertical arrangement of these fibres. An 

 understanding of the looseness and elasticity of the skin is of much practical im- 

 portance to the surgeon in certain operations. 



When the traction is slow as over a slow-growing tumour, or over the abdomen and breasts in 

 pregnancy, the skin may be stretched to a very considerable degree. In these cases there are 

 often produced short parallel reddish streaks which when the stretching is relieved are re- 



