TISSUES, ORGANS, AND SYSTEMS. 103 



The physiological importance of the smooth muscles lies in their con- 

 tractile power ; in consequence of which they afford 

 considerable assistance to the functions of the differ- Fig. 34. 



ent viscera. The development of their elements takes 

 place simply by the elongation of rounded cells, the 

 membranes and contents uniting into a homogeneous 

 soft substance. The nutrition of the smooth muscles 

 would seem to go on very actively, according to the 

 later investigations upon the fluid which bathes them, 

 which, according to Lehmann, has most generally a 

 distinctly acid reaction, and together with lactic, 

 acetic, and butyric acid, contains creatin and inosit ; 

 and the same conclusion may be deduced from the fre- 

 quent occurrence of physiological (in the uterus) and 

 pathological hypertrophies and atrophies of them. 

 Whether smooth muscles are regenerated, or whether 

 loss of their substance is replaced by a similar tissue, 

 is unknown ; on the other hand, new formations of 

 them appear to occur in uterine tumors. 



The smooth muscular fibres never form large iso- 

 lated muscles in the human body ; as, for example, 

 is the case in the genito-rectal muscles of mammalia, 

 but exist either scattered in the connective tissue, or 

 in the form of muscular membranes. In both cases 

 the bundles are either parallel or interwoven into net- 

 works. Their distribution is as follows : 



1. In the Intestinal canal the smooth muscle 

 forms : first, the tunica musculosa from the lower 

 half of the oesophagus, where smoth bundles are still 

 mingled with transversely striated fibres, as far as 



the sphincter ani internus : secondly, the muscular layers of the mucous 

 membrane, from the oesophagus to the anus: and thirdly, scattered mus- 

 cular bundles in the villi. 



2. In the Respiratory organs, a layer of smooth muscles appears in 

 the posterior wall of the trachea, and accompanies the bronchia?, even 

 to their finest ramifications, as a complete circularly fibrous membrane. 



3. In the Salivary glands, this tissue is found solely in Wharton's 

 duct ; and here only scantily, and forming an incomplete coat. 



4. The Liver has a perfect muscular layer in the gall-bladder, and 

 scattered smooth muscles also in the ductus choledochus. 



5. The Spleen has this kind of muscle in many animals in its outer 



FIG. 33. Muscular fibre-cell from the small intestine of man. 



FIG. 34. Muscular fibre-cell from the fibrous investment of the spleen of the dog; magni- 

 fied 350 diam. 



