Muscle and Electric Organs 577 



muscle functions purely isotonically or isometrically, but individual muscles 

 can approach these conditions. 



2. Muscles arranged around hollow structures, lacking strict origins and 

 insertions, one portion of the muscle inserting into and hence pulling 

 on another portion of itself. These are predominantly "holding" muscles. 

 Among the vertebrates the smooth muscles of the bladder, the ureter, the 

 stomach, and the intestine, and striated muscle such as the sphincter ani and 

 esophageal muscle and the heart are circular muscles, as are also the body 

 wall muscles of annelids, holothurians, and other "hohlorganartige Tiere," as 

 Jordan calls them. In general, muscles of hollow structures are slower than 

 phasic inserted muscles; they also occur in pairs, e.g., the circular and longi- 

 tudinal muscles of the bodv wall of annelids, producing reciprocal move- 

 ments. These muscles contract against sacs of fluid rather than against skele- 

 tal or cuticular structures. 



Muscles are also classified as tetanic (rapidly contracting) and tonic (hold- 

 ing).*^ Frequently the same muscle can perform both functions. Some 

 muscles with fixed attachments are tonic, as the clam adductors, but in gen- 

 eral the tonic properties are more common among the muscles enclosing 

 cavities. 



Muscles of both of the above categories are under reflex control, the 

 tetanic more so than the tonic. Within a muscle the motor nerve fibers 

 branch extensively among muscle fibers. A group of muscle fibers together 

 with the motoneurone which innervates it constitutes a motor unit. In mam- 

 mals several hundred muscle fibers may be served by one motor nerve fiber. 

 In crustaceans and in some molluscs one motor nerve fiber may supply an 

 entire muscle (see p. 596). The extent to which visceral muscles are 

 organized in motor units is not clear. The smooth muscles of vertebrate 

 blood vessels and pilomotors behave as if they were arranged in motor 

 units.^** By contrast, in intestinal, uterine, and cardiac muscle, and probably 

 in the molluscan foot, conduction occurs from muscle cell to muscle cell 

 or by a nerve plexus. Many muscles receive several types of innervation, 

 separate motor innervations for fast contractions, for slow contractions, and 

 sometimes for inhibition. 



If movement is to be useful not only must it be performed at a suitable 

 rate, but it must be capable of gradation in rate and strength by one or more of 

 the following mechanisms: (1) The grading of movement in a muscle organ- 

 ized in units is regulated from the central nervous system by varying the num- 

 ber of units active. (2) Activity in all types of muscle is graded in part by 

 variation in frequency of motor stimulation of individual units, high fre- 

 quencies eliciting strong contractions. (3) Another method of gradation 

 is possible wherever multiple innervation occurs; different nerve fibers cause 

 contractions of diff^erent rates, or cause either local or all-or-none contrac- 

 tions, and inhibitory fibers diminish the effect of excitatory fibers. 



TIME RELATIONS OF MUSCLE 



Histological Correlations: Electron Microscopy of Muscles. Vertebrate 

 muscles have long been classified as striated, cardiac, and smooth, and it 

 has long been recognized that the fastest muscles are striated. Muscles 



