270 



THE SKIN 



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, Euffini, and 

 the G olgi-Mazzoni corpuscles (see Chapter VI). 



Small bundles of smooth muscle fibers which form the arrectores 

 pilorum 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 subcutaneous tissue. 



At the level of the vascular 

 plexuses between the corium 

 and the tela subcutanea, Hagg- 

 quist (Anat. Anz., 45, 2, 1913) 

 has described a thick bundle 

 of smooth muscle not previous- 

 ly recognized. It lies directly 

 beneath a cold spot, and is not 

 found in skin lacking cold 

 spots. The muscle is believed 

 to contract reflexly when a cold 

 object is placed on the skin and 

 constrict the local blood supply. 



DEVELOPMENT AND 

 * s GROWTH OF THE SKIN 



The skin may be said to 

 arise with the first differen- 

 tiation of the embryo into 

 its three germ 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 superficial periderm (epitrichium) , and a deeper germinal 



layer. 



FIG. 265. THREE EARLY STAGES IN THE 



HlSTOGENESIS OF THE SKIN. 



A, single-layered epidermis from dorsal 

 body wall of a 5 mm. human embryo; B, two- 

 layered epidermis, from dorsal body wall of 

 13 mm. human embryo; C, multiple-layered 

 epidermis from nose of 21 mm. pig embryo. 

 Per, periderm (epitrichium) ; Epi, epidermis; 

 Mes, mesenchyma differentiating into the 

 derma. X 750. 



