1286 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



the surface of the corium and joined together by fine fibrils; more superficially they become 

 round or polyhedral. These cells together with several more superficial layers form a stratum 

 from which the other cells of the epidermis are developed and which therefore are Icnown as the 

 stratum germinativum (Malpighii). The cells in the superficial part of this stratum, in some 

 situations, have a granular appearance forming a layer which is called the stratum granulosum. 

 Superficial to this there is, also only in some places, a layer in which the cells are somewhat 

 indistinct and transparent, and therefore known as the stratum lucidum. This is a transition 

 between the softer and more opaque stratum germinativum and the firmer and more transparent 

 superficial layer formed of large, flattened, dry, horny cells, known as stratum corneum. 



In general the stratum germinativum is thicker than the stratum corneum. In certain 

 parts as the face, the back, the back of the hands and feet, the two layers are equal in thickness. 

 In other regions, as in the volar surface of the hands and plantar surface of the feet, the stratum 

 corneum is much thicker than the stratum germinativum varying from two to three or even five 

 times as thick. This increased thickness of the stratum corneum is not due to pressure alone 

 as it is well marked in the foetus, but it is not improbable that pressm-e may stimulate the 

 further growth of the cells. 



Where the papillae of the corium are arranged in rows as on the volar surface of the hands 

 and the plantar surface of the feet, the epidermis is molded to these so as to appear as ridges 

 on the surface, already described as cristae. In most other places the irregularities of the papillae 

 of the corium do not show on the surface. At short and regular intervals on the cristae are 

 notches and transverse furrows which mark the openings of the sweat glands. 



The separation of the epidermis from the corium by the accumulation of serous fluid between 

 the layers is knowTi as a bhster. Sometimes it is only the separation of the superficial layers 

 from the deeper layers of the epidermis. 



The skin is regenerated after a bhster or a wound by growth of the cells of the stratum ger- 

 minativum. It is probable that cells of the superficial layers take no part in this. Therefore in 

 skin grafting the surgeon in order to transplant the cells of the stratum germinativum usually 

 includes aU the layers of the epidermis and the extreme tips of the papillae of the corium as shown 

 by the minute bleeding points left on the surface from which the graft has been cut. 



The pigment which gives the main colour to the skin is caused by the accumulation of pig- 

 ment granules, melanin, in the deepest cells of the stratum germinativum. It does not occur 

 imtil after the sixth month of foetal life and develops chiefly after birth. The blackness of the 

 skin of the negro depends almost entirely upon this pigment. Pigment granules are also found 

 to a less extent in more superficial cells and sometimes in the corium. 



Development of the epidermis. — The epidermis is derived from the ectoderm, in early 

 embrj^os appearing as a double stratum of cells, the superficial layer of which is known as the 

 epitrichium or periderm, the deep layer becomes the stratum germinativum. By multiplica- 

 tion of the deep cells a number of layers are produced and the more superficial cells tend to 

 assume the adult characteristics. At about the sixth month of fcetal life the epitrichial layer 

 finally disappears. The surface layers are cast off and mixing with the secretion of the cutaneous 

 glands form a yellowish layer over the surface of the skin of the fa?tus, the vernix caseosa. 



Growth continues throughout life. New cells are formed in the deeper layers pushing the 

 older cells toward the surface. The character of the cells changes as they approach the surface, 

 the change being quite abrupt at the level of the stratum lucidum. As the form of the cells 

 changes, chemical and physical alterations of their contents occur. In most places the super- 

 ficial cells are represented by thin scales but in the palms and soles the cells are somewhat swol- 

 len. The superficial cells are being constantly thrown off and replaced by deeper ones. 



The corium (cutis, cutis vera, derma) is a fibrous vascular sheath composed 

 of interwoven bundles of connective-tissue fibres intermixed with elastic fibres, 

 connective-tissue cells, fat, and scattered unstriped muscle-fibres. It is traversed 

 by rich plexuses of blood-vessels, lymph-vessels, and nerves, and encloses hair- 

 bulbs and sebaceous and sudoriferous glands. It varies in thickness from .3 

 mm. to 3.0 mm. or more, usually ranging from .5 to 1.5 mm. It is to this layer 

 that the strength and elasticity of the skin are due and it is also only this layer 

 which when properly cured we know as leather. 



The superficial layer of the corium is of finer, closer texture, free from fat, and 

 forms a multitude of eminences called papillae corii (figs. 1040, 1045, 1046) which 

 project into corresponding depressions on the deep surface of the epidermis. 

 For this reason this part of the corium although ])ut indistinctly separated from 

 the deeper layer is called the corpus papillare. 



Some of the papilla; contain vessels, others nerves, hence they are known as vascular or 

 tactile papillae. They are very closclj' set, varying considerably in number in different parts of 

 the body from 3G to 130 to a .square millimetre, and it has been estimated that there are about 

 150 million papilla; on the whole surface. They also vary greatly in size not only in different 

 regions but in the same region, being from .03 to .2 mm. or more in height. 



The deei)er layer of the corium, the tunica propria (stratum rcticulare), is 

 composed of coarser and less compact bands of fibrous tissue intermingled with 

 small fat lobules. The fibrous and elastic tissue is arranged for the most part in 

 intercrossing }>undles nearly parallel to the surface of the skin. 



The bundles rurming in some directions are usually more strongly developed and more 

 numerous than those in others but tlic direction of the strongly developed bundles varies in 



