Chap. VII] MUSCULAR TISSUE 93 



and give up to the lymph the waste substances that are the result 

 of their activities. 



In order to understand the nerve supply it is necessary to 

 become familiar with a few facts regarding the nervous system. 

 The name nervous system includes all the structures in the 

 body that are made of nerve tissue. For purposes of study 

 the nervous system is arbitrarily divided into the central ner- 

 vous system, and the sympathetic system. The central nervous 

 system consists of the brain, the spinal cord, and their nerves. 

 The sympathetic system consists of masses of nerve cell-bodies 

 called ganglia, and the nerves connected with "them. These 

 two systems differ slightly in function, but are intimately con- 

 nected and are really interdependent. Most of the nerves that 

 are connected with the skeletal muscles belong to the central 

 nervous system, but the majority of those supplying the visceral 

 muscles belong to the sympathetic system. Nerves that carry 

 impulses from the periphery (in this connection the muscles) to 

 the brain, spinal cord or ganglia, are called afferent. Nerves that 

 carry impulses to the periphery from the brain, spinal cord or gang- 

 lia, are called efferent. Afferent nerves that are connected with 

 muscle fibres are spoken of as sensory. Efferent nerves that end 

 in muscle fibres are spoken of as motor. For the purposes of this 

 chapter, each of the terms in these pairs, i.e. motor and efferent, 

 sensory and afferent, may be considered interchangeable. It should 

 be remembered, however, that the terms efferent and afferent are 

 applied to other types of nerves besides motor and sensory nerves.^ 



Differences in, and results of, contraction. — Skeletal muscle 

 is essentially a quick-acting tissue. It contracts quickly and re- 

 laxes promptly. Even prolonged contractions are compounded 

 of successive twitches that follow each other too rapidly to permit 

 of relaxation. In sharp contrast to this the contractions of vis- 

 ceral muscle develop slowly, are maintained for some time, and 

 fade out slowly. In addition to the contractions that are the 

 result of stimulation from the nerves, visceral and cardiac tissue 

 are also automatic. This means that there is a tendency to rhyth- 

 mic contraction and relaxation inherent in the tissue itself. 



Contractions of all kinds of muscular tissue cause a chemical 

 change in the substance of the muscle fibre. The complex sub- 



' For a more detailed description, see Chap. XIX. 



