SECTION X 

 OTHER FORMS OF CONTRACTILE TISSUE 



SMOOTH OR UNSTRIATED MUSCLE 



THE little we know about the physiology of unstriated muscle is derived 

 chiefly from experiments on the intestine, ureter, bladder, and retractor 

 penis.* This tissue differs from voluntary muscle in containing numerous 

 plexuses of nerve fibres (non-medullated) and ganglion cells, so that in all 

 our researches it is difficult to be certain whether the results are due to the 

 muscle fibres themselves, or to the nerves and nerve cells which are so inti- 

 mately connected with them ; especially as we have as yet no convenient 

 drug like curare, by aid of which we might discriminate between action on 

 muscle and action on nerve. 



The differences between unstriated and voluntary muscle, although at 

 first sight very pronounced, on further investigation prove to be in most 

 cases differences of degree only, qualities and reactions which are marked 

 in involuntary muscle being also present in a minor degree in the more 

 highly differentiated tissue. 



The contraction of smooth muscle is so sluggish that the various stages 

 of latent period, shortening, and relaxation can be easily followed with the 

 eye. The latent period may be from 0-2 to 0-8 second, and the contraction 

 may last from three seconds to three minutes. 



Smooth muscle preserves many of the properties of undifferentiated 

 protoplasm, especially an automatic power of contraction, which is regulated 

 by the condition of the muscle. Thus whereas the voluntary muscle is 

 intimately dependent on its connection with the central nervous system, 

 and in the absence of this is reduced to a flabby inert tissue, the smooth 

 muscle, isolated from all its nervous connections, presents in many cases 

 rhythmic contractions, and can carry out a peripheral adaptation to its 

 environment. These rhythmic contractions are almost invariably observed 

 if the muscular tissue be subjected to a certain amount of tension, after 



* The retractor penis, which is found in the dog, cat, horse, hedgehog (but not in 

 rabbit or man), is a thin band of longitudinally arranged unstriated muscle, which 

 is inserted at the attachment of the prepuce, and is continued backwards in a sheath of 

 Connective tissue to the bulb, where it divides into two slips, which pass on either side 

 of the anus. It is innervated from two sources, the motor fibres being derived from 

 the lumbar sympathetic and running to the muscle in the internal pudic nerve, while 

 the inhibitory fibres run in the pelvic visceral nerves (nervi erigentes) and are derived 

 from the second and third sacral nerve-roots. 



. 243 



