DEVELOPMENT OF THE SKIN AND ITS APPENDAGES. 861 



ducts open into the superficial parts of the hair follicles (Fig. 733) ; the number of 

 glands associated with each follicle varies from one to four. On the labia minora 

 and mammary areolse they open on the surface of the skin independently of hair 

 follicles, and in the latter situations undergo great enlargement during pregnancy. 

 The deep extremity of each gland expands into a cluster of oval or flask-shaped 

 alveoli, which are surrounded by a basement membrane, and filled with polyhedral 

 cells containing oil droplets. By the breaking down of the superficial cells, their 

 oily contents are liberated as the sebum cutaneum and discharged into the hair 

 follicle, whilst the deeper cells undergo proliferation. The size of the gland bears 

 no proportion to that of the hairs, since they are very large in the minute hair 

 follicles of the foetus and newly born child, and also in the follicles of the rudimen- 

 tary hairs of the nose and certain parts of the face. 



Bundles of non-striped muscular fibre are associated with the hair follicles, and 

 are named the mm. arrectores pilomm. Attached to the deep part of the hair follicle, 

 and forming with it an acute angle, they pass outwards close to the sebaceous 

 glands, and end in the papillary layer of the corium. They are situated on the 

 side towards which the hair slopes, so that, on contraction, they diminish the 

 obliquity of the hair follicle and render the hair more erect, and, at the same 

 time, compress the sebaceous glands and expel their contents. The condition of 

 " goose-skin " is caused by the contraction of these slender muscles. 



Arthur Thomson suggests that the condition of curly hair is produced by the contraction of 

 the mm. arrectores pilorum. Straight hair is thick and cylindrical ; curly hair is flat and ribbon- 

 like. When the arrector muscle contracts, the thick rounded hair resists the tendency of the 

 muscle to bend it, while the flat hair, not sufficiently strong to resist the strain of the muscle, 

 becomes bent, and this is probably the explanation why the follicle assumes the curved form 

 characteristic of the scalp of a bushman. The sebaceous gland lies in the concavity of the bend 

 between the follicle and the muscle, and forms a mass of greater resistance, around which the 

 follicle may be curved by the contraction of the muscle. The cells at the root of the hair 

 accommodate themselves to the curved follicle, and, becoming more horny as they advance to 

 the surface, retain the form of the follicle in which they are moulded. 



Glandulae Sudoriferse. Sudoriferous or sweat glands are found in the skin 

 of nearly every part of the body ; they are relatively few in number on the back 

 of the trunk, but are very plentiful on the palms and soles, where they open on the 

 summits of the curved ridges. Each consists of an elongated tube, the deeper portion 

 of which forms its secretory part, and is coiled in the subcutaneous tissue or deep 

 part of the corium in the form of an ovoid or spherical ball, termed the corpus 

 glandulse sudoriferse (O.T. glomerulus) (Fig. 733). The superficial part of the tube, or 

 ductus sudoriferus, extends through the corium and epidermis, and opens on the 

 surface by a funnel-shaped orifice, the poms sudoriferus ; where the epidermis is thick 

 the duct is spirally coiled. The bodies of the glands, as a rule, vary in diameter 

 from 0*1 to 0'5 mm., but in the axillse they are much larger, and may measure 

 from 1 to 4 mm. Each is surrounded by a capillary network and by a capsule of 

 connective tissue, inside which is a homogeneous basement membrane. The lumen 

 of the tube is lined with a layer of nucleated, granular, and striated, columnar, 

 or prismatic epithelial cells, between the deep extremities of which and the 

 basement membrane is a layer of non-striped muscular fibres, the long axis of which 

 is more or less parallel with that of the tube. The excretory ducts are devoid of 

 muscular fibres, and consist of a basement membrane lined by two or three layers of 

 polyhedral cells, which are covered, next the lumen of the duct, with a thin cuticle. 



The glandulse ciliares, at the margins of the eyelids, and the glandulse ceruminosse 

 of the external acoustic meatus, are modified sudoriferous glands ; the former are, 

 however, not coiled, while the cell protoplasm of the latter contains yellowish 

 pigment, and their gland ducts, in the foetus, open into hair follicles. 



DEVELOPMENT OF THE SKIN AND ITS APPENDAGES. 



Skin. The vascular and sensitive corium is developed from the mesoderm, the cells 

 of which, immediately underlying the ectoderm, have, by the second month of foetal life, 

 become aggregated together and flattened parallel to the surface of the embryo. By the 

 third month they are seen to form two layers, the superficial of which becomes the 



