204 THE SKIN 



which are derived from the insertions of the mimetic muscles. 

 The corium of the scrotum (where it forms the tunica dartos), of 

 the penis, perineum, and areola of the nipple contain much smooth 

 muscle. 



SUBCUTANEOUS TISSUE 



The subcutaneous tissue (tela subcutanea, pannicnlus adiposus, 

 subcutis) consists of bands and septa of fibrous connective tissue 

 which extend from the deeper margin of the derma to the under- 

 lying fasciae of the muscles, the periosteum of the bones, etc. The 

 direction of these fibrous bundles is very variable. The more 

 nearly parallel to the cutaneous surface the fibre bundles are, and 

 the looser the meshes which they form, the greater is the mobility 

 of the skin. 



The meshes of the subcutaneous network are occupied by 

 lobules of adipose tissue. The subcutaneous tissue contains the 

 main nerve trunks and larger blood vessels of the skin, the larger 

 sudoriparous and sebaceous glands, and the coarser hair follicles. 

 It also, together with the deeper part of the derma, contains the 

 nerve end organs of Pacini, KuflBni, and the Golgi-Mazzoni cor- 

 puscles.* 



Small bundles of smooth muscle fibres which form the arrector 

 pili muscles take origin from the deeper surface of the corium and 

 are inserted into that portion of the hair follicle which is embedded 

 in the subcutaneous tissue. These fusiform or columnar muscle 

 bundles are found in connection with all the hairs, but in the scalp 

 they are most highly developed and lie most deeply in the sub- 

 cutaneous tissue. 



DEVELOPMENT AND GROWTH OF THE SKIN. The skin 

 may be said to arise with the first differentiation of the embryo 

 into its three germinal layers. The ectoblast, which is at first a 

 single cell layer, becomes a double layer by the end of the first 

 month. It continues to increase in thickness until by the end of 

 the second month it can be differentiated into two layers, a super- 

 ficial or epitrichium, and a deeper germinal layer. 



The epitrichium forms a layer of peculiar dome-shaped cells 

 with flattened margins and a vesicular center. It continues to 

 form the superficial layer of the epidermis until about the sixth 

 month, when it is lost by desquamation. The germinal layer con- 

 sists of a deep stratum of cylindrical cells and one or two superfi- 



* See Chapter IX. 



