34 



PSYCHOBIOLOGY 



the skin, or the mucous membrane, the ends of the fibers may be branched 

 or pointed, the perimysium being prolonged as elastic fibers which be- 

 come continuous with the connective tissue in the skin or membrane. 



Smooth muscle develops from mesenchymal cells. It is found sur- 

 rounding the large blood vessels and lymphatic ducts, the intestinal canal 

 and the ducts* of the principal gland opening into it, the large respiratory 



Fig. 23. Smooth muscle in longitudinal section of small intestine. Magnified 350 

 diameters. (Bailey, Histology.) The inner circular layer (at top), traversely cut, 

 and the outer longitudinal layer are shown with the intermuscular septum of con- 

 nective tissue between. The cross-section of a small artery is visible in the septum. 



tubes and the passages and ducts of the genito-urinary system, and subcu- 

 taneously in connection with hairs. It may for convenience (although 

 not with exact accuracy) be called visceral muscle, in contradistinction to 

 skeletal muscle. 



The fibers of smooth muscle are elongated spindle-shaped cells, each 

 with a single nucleus centrally located [Fig. 24]. The fibers vary in 

 length from 20/x to 500 p and are typically about 5 /a in diameter. Within 



Fig. 24. Isolated smooth muscle cells from human small intestine. Magnified 400 

 diameters. (Bailey, Histology.) The centrally located nuclei are visible as oval, 

 darker areas. 



these cells ' coarse ' longitudinal fibers, composed entirely of anisotropic 

 (»'. e., doubly refracting) substance, develop near the surface. These 

 border fibrils are believed to be continuous from cell to cell. Surrounded 

 by the border fibrils are fine inner fibrils, which separate to pass around 



