SMOOTH MUSCLE 



101 



largest of these fibers is seen in the pregnant uterus of mammals at term, 

 when the fibers attain an enormous size. 



The smooth muscle cells in the walls of the calf's bladder show an 

 intercellular fibrous connective tissue. This can be differentiated from 

 the muscle substance by staining, and it appears as a fibrillar material 

 in our figure. It is produced, as was the similar substance of the earth- 

 worm's gizzard cell, by the muscle cell. 



A section of any tubular portion of 

 the digestive tract of a mammal em- 

 bryo of the right age will show the 

 origin of smooth muscle. Lewis has 

 described the process in the esophagus 

 of a pig of 8 mm. (Fig. 100). The 

 organ consists at first of an inner 

 stratified epithelium resting on a mes- 

 enchymal layer of primitive connective 

 tissue whose cells have not yet begun 

 to form the fibrils. When the fibrils 

 do appear, they are myo-fibrils, all 

 lying in the same direction and filling 

 the cells up until they are the familiar 

 smooth muscle cells. 



These cells do not appear through- 

 out the mesenchyme, but in two layers 

 of it, the inner of which produces fibers 

 that run in a circular direction, while 

 the outer one lays them down at right 

 angles to these and parallel to the axis of the tube. Both these layers 

 are removed from the epithelium by a third, which remains a true con- 

 nective tissue, becoming a layer of soft, fibrous connective tissue in the 

 adult state. Even while forming the muscle fibers, the cells remain 

 partly connected by processes which, at this stage, are true intercellular 

 processes. This clearly appears in the transections of the longitudinal 

 or outer layer of muscle cells, and these bridges form the fine inter- 

 cellular connective tissue, afterwards retreating into the cell and becom- 

 ing part of its sarcoplasm. 



Technic. Smooth muscle differs from the other kinds only in the 

 way that several special processes can be applied to it. Its cells are 

 most easily isolated by maceration and teasing, even after they have 

 been fixed in several fluids. Such preparations should be supplemented 

 by well-stained and thin sections in which the finer cytological details 

 can alone be brought out. There are stains that are specific for the 

 smooth muscle fiber when there is doubt as to whether it is muscle or a 

 connective-tissue structure (see LEE). 



atransvers e section of 



the digestive tract of an embryonic pig. 

 mus.fi., developing smooth muscle fibers. 

 (From "STOHR'S Histology," by LEWIS.) 



