1074 THE SKIN 



some of wliich, as those on the chin and cheeks and at the uhiar liorder of tlie 

 palm, correspond to the points of attachment of muscular fibres into the deep sur- 

 face of the skin, and are made evident by the contraction of tliese fibres; while 

 others depend upon the attachment of the skin by fibrous bands to the l>ony emi- 

 nences, as of the elbow, shoulder, vertel)ra3, and posterior iliac spines, and are seen 

 only when the subcutaneous adipose tissue is well developed. 



The skin is separated from the superficial bones and muscles, etc. , by a subcu- 

 taneous structure, comprising white fibrous and elastic tissues, fat, and, in certain 

 situations, layers of striped or unstriped muscle. The subcutaneous fil)rous tissue 

 over the greater part of the liody is arranged in two planes, known as the superficial 

 and deep layers of the superficial fascia, separated by a layer of fat. It is closely 

 blended with the deep aspect of the skin, but only loosel}' united to the deep -fascia 

 investing the nuiscles, and permits the cutaneous and subcutaneous structures to 

 move more or less freely over the deep parts. In some situations, moreover, where 

 the integument is exposed to repeated friction over sul)jacent l)ones or other hard 

 structures, its movements are facilitated by the development of sac-like intersjnices 

 in the subcutaneous tissue, called bursse. Exceptions to this rule of mobility of 

 integument are found in the head and face, where the skin is intimately con- 

 nected with the subjacent muscular and tendinous tissues, and in the palms and 

 soles, where it is firmly attached to the deep fascia. 



The subcutaneous fat varies considerably in amount and character in different 

 parts of the body. It is entirely absent on the penis and scrotum and is largely 

 developed over the nates, palms, and soles, where it serves as pads or cushions. In 

 the scalp it appears as a single uniform layer of ovoid lobules between the derm 

 and the aponeurosis of the occipito-frontalis muscles; on other parts of the surface 

 it is somewhat unequally distributed, and shows a tendency to accumulate in appar- 

 ent disproportion in certain localities, as on the abdomen, over the symphysis 

 pubis, about the mammae in females, etc. Everywhere, except perhaps on the 

 scalp, it may undergo rapid and visible increase or decrease under the influence of 

 changes of nutrition. 



The subcutaneous planes of striped muscle are comparatively scanty in man 

 when compared wuth the great panniculus carnosus of the lower mammalia. They 

 are best represented by the platysma myoides on the neck. Unstriped muscular 

 fibre, present in all parts of the skin as an appendage to the hair-follicles and 

 SL'l)aceous glands, is found also in some parts of the sul^cutaneous tissue, as in the 

 scrotum and perinseum, around the anus, in the mammilla, and beneath the 

 mammary areola. 



Structure of the skin. — The skin is separable into two distinct layers — a 

 deeper, the derm, cutis vera, or corium; and a more superficial, the epiderm, 

 cuticle, or scarf-skin. The latter, morphologically representing the epiblast, is a 

 purely epithelial cell-growth, while the derm, which is developed from the meso- 

 blast, is of highly complex organisation. 



The derm is composed of elastic fibrous tissue intermixed with fat vesicles, and 

 ])undles of un.striped muscular fibres, traversed by a rich })lexus of blood and 

 lymph-vessels and of nerves, and enclosing hair l)ull)s and seliaceous and sudor- 

 iparous glands. The su]>erficial layers are of finer structure, free from fat, and 

 form a multitude of eminenc^es, called papillae, Avhich project ujiwards into corre- 

 sponding depressions in the sul)stance of the epiderm. The fil^rous bands compos- 

 ing the deeper layers are coarser and less compact, intermingled with fat lobules, 

 and pass without any definite line of demarcation into the subcutaneous tissue. 

 The muscular fibres ap])ear as appendages to the hair and its sebaceous follicles, 

 and are known as arrectores pilorum. Two or three bundles arc attached to 

 each hair-i^heath below tlie scl)aceous gland opening into the follicle, and are so 

 arranged as to be ca])alile of compressing the acini of the gland, and of rendering 

 the direction of the hair-shaft somewhat more per])en(Hcular to the surface from 

 which it emerges. This latter action ])roduces the ' bristling of the hair' which is 

 believed to be one of the manifestations of extreme terror, but which occurs in many 

 persons in association with mental excitement of various kinds. 



The fibrous tissue runs for the most part in l)undles parallel to the surface, 

 intercrossing at various angles in a manner that appears to be regulated upon a 



