MUSCLE 523 



of a muscle is continuous with that of its tendon, and that in 

 its turn, as we have seen, with that of bone, so that the contraction 

 of the muscle is transmitted directly to the bone which it is to 

 move. 



The syncytial muscle fibre is produced by the division of the 

 nucleus of a single connective tissue cell. 



Visceral, smooth, plain or involuntary muscle is chiefly associ- 

 ated with the alimentary canal, the excretory system and other 

 internal organs. Its unit, the smooth muscle fibre (Fig. 408) is 

 a single spindle-shaped cell, without cross-striations. Fibrillse 

 can be made out on treatment, but there is no true sarcolemma. 

 Smooth fibres are smaller than striated fibres, not more than half a 

 millimetre long. They are usually arranged in sheets or tubes 



Fig. 409. — Diagram of cardiac muscle, showing nuclei, and side branches which 

 give syncytial continuity. — From Le Gros Clark, The Tissues of the Body, 

 3rd edition, 1952. Clarendon Press, Oxford. 



rather than in bundles, and their period of contraction is longer 

 than that of striped muscles. There is a double motor innervation 

 from sympathetic and parasympathetic nerves, the former 

 inhibiting and the latter promoting contraction. Nerve cell bodies, 

 as well as fibres, are present in the muscles of the gut, so that 

 there are networks or plexuses. As in skeletal muscle, the terminal 

 fibres penetrate the cytoplasm. Smooth muscle shows great 

 abihty to contract without nervous stimulation, which is pre- 

 sumably why it needs two sets of nerves. 



The muscle of the heart, or cardiac muscle, is unique, and in 

 many ways it is intermediate between smooth and striped muscle. 

 The fibres branch and join, so that the whole mass of muscle 

 forms a syncytium. Nuclei, cross-striations and fibrils can be 

 seen, as well as cross walls which look like, but are probably not, 

 cell walls, in spite of the fact that they separate the nuclei (Fig. 

 409). The heart in the embryo begins contracting before it has 



