CH. vi.] DEVELOPMENT OF MUSCLE. 95 



which is the principal cause of the wrinkling and contraction of 

 the scrotum on exposure to cold. It occurs also in the skin 

 generally, being found surrounding the secreting part of the sweat 

 glands and in small bundles attached to the hair follicles ; it also 

 occurs in the areola of the nipple. It is composed of long, fusi- 

 form cells (fig. 114), which vary in length, but are not as a rule 

 more than -5-^-5- inch long. Each cell has an oval or rod-shaped 



Fig. 114. Muscular fibre-cells from the muscular coat of intestine highly magnified. 

 Note the longitudinal stria! ion, and in the broken fibre the sheath is visible. 



nucleus. The cell substance is longitudinally but not transversely 

 striated. Each cell or fibre, as it may also be termed, has a deli- 

 cate sheath. The fibres are collected into fasciculi, and united 

 by cementing material, which can be stained by silver nitrate. 

 This intercellular substance is bridged across by fine filaments 

 passing from cell to cell. 



The nerves in involuntary muscle (both cardiac and plain) do 

 not terminate in end-plates, but by plexuses or networks, which 

 ramify between and around the muscular fibres. 



Development of Muscular Fibres. 



All muscular fibres (except those of the sweat glands which 

 are epiblastic) oiiginate from the mesoblast The plain fibres 



