HAIR. 



319 



Blood vessel. 



Hair canal. 



is always below the sebaceous gland and on the lower surface of the hair 



as shown in Fig. 370. 



The hairs which cover 



the body of the embryo 



and which to a variable 



extent persist after birth, 



are soft and downy; 



they are known as lan- 



ugo. Arrector muscles 



are absent from the lan- 



ugo of the nose, cheeks 



and lips, and also from 



the eyelashes (cilia) and 



nasal hairs (vibrissae). 



In describing the de- 

 velopment of hairs it has 

 been stated that a hair 

 consists of a papilla, bulb, 

 and shaft; and that the 

 part of the shaft beneath 



Hair matrix. 



Outer sheath. 





FIG. 369. VERTICAL SECTION OF THE SKIN OF THE BACK OP A 

 nnH cViaft- and that flip HUMAN FETUS OF FIVE AND A HALF MONTHS. X 120. 



snail, d,n The staining with iron haematoxylin has .made the horny parts so 



black their details are invisible. 



Root ' 



the epidermis is covered with a connective tissue sheath, an outer epithelial 



sheath, and below 

 the sebaceous 



shaft. ^ gland, with an in- 



ner epithelial 

 sheath. The finer 

 structure of the 

 shaft and its 

 sheaths is shown in 

 the cross section, 

 Fig. 371, and the 

 longitudinal sec- 

 tion, Fig. 372; it is 

 described in the 

 following para- 

 graphs. 



The connective 

 tissue sheath is de- 

 rived from the co- 

 rium. It is found 



about the larger hairs where it may be divisible into three layers. The 



Sebaceous gland. 



Arrector pili 

 muscle. 



Epithelial 

 sheaths. 



Connective tissue, 

 sheath. 



Bulb. 



Papilla. 



Fat cells. 

 FIG. 370. FROM A THICK SECTION OF THE HUMAN SCALP. X 20. 



