SMOOTH MUSCLE 97 



SMOOTH MUSCLE TISSUES 



Smooth muscle originates in the embryo as a specialization of mesen- 

 chymal cells. It appears as a set of unicellular fibers, all lying parallel 

 for mutual support in their efforts. Sometimes they are crossed or inter- 

 laced, but in this case only certain sets act together to effect certain mo- 

 tion. All the fibers in one direction may contract to cause a shortening 

 of some part of the body ; again, all the fibers lying in several directions 

 in one plane or in two given planes may contract to force an extension 

 of a part of the body in another direction or plane. In form, these fibers 

 are almost always elongated spindles tapering into thin ends. Very 

 rarely these ends may be branched into two or three main branches, as is 

 sometimes found in the young mammalian aorta, or they may give off 

 many fine side branches, as in the smooth muscle cells of the heart of 

 some mollusks. These side branches are not contractile, however. 



The smooth muscle fiber is always formed in a single cell and has a 

 single nucleus. We recall no case where it has the syncytial multiplicity 

 of nuclei found in the large striated muscle cells. This nucleus may 

 appear inside of the fiber or outside of it. This idea may be as well 

 expressed by saying that the myo- fibrils are distributed around the 

 nucleus or to one side of it. 



The smooth muscle cell, like all muscle cells, owes its contractile 

 power to the development of a varying number of myo-fibrils in its 

 cytoplasm. These fibrils may be very few and appear unimportant in the 

 make-up of the cell, or they may be many, and at first sight be all of it. 

 Perhaps the most ordinary method of their appearance is in the entire 

 periphery of the cell where they form a thin layer (see Fig. 97), or a thick 

 layer that occupies most of the room in the cell body (see Fig. 99). When 

 the fibrils appear in bundles or a single bundle in one side of the cell- 

 body, their development, if large, leaves the cell body as an apparent 

 appendage on one side. This is seen in some low forms, especially 

 well in the nematode worms (see Fig. 95). 



The smooth muscle fiber is generally used by animals that move 

 slowly or in parts of the anatomy of more active animals where a slower 

 motion not directly controlled by the will is needed. There are a few 

 examples, however, of organisms which are noted for their swiftness and 

 beautiful muscular activity, and yet have nothing but smooth muscle 

 to perform their actions with. The squid is such an example, and its 

 muscle is also absolutely under the control of the larger and central 

 nerve centers, making it "voluntary" in a proper sense. 



A contractile fiber from Euspongia officinalis will show a very low 

 form of smooth fiber with very few fibrils and much granular sarcoplasm. 



