MUSCLE 627 



Lying deeper than the peculiarities of individual muscles, 

 muscular tissue has certain common properties physical, 

 chemical, and physiological. The biceps muscle flexes the arm 

 upon the elbow, and the triceps extends it. The external rectus 

 rotates the eyeball outwards. The intercostal muscles elevate 

 the ribs. The sphincter ani seals up by a ring-like contrac- 

 tion the lower end of the alimentary canal. These actions are 

 very different, but the muscles that carry them out are at bottom 

 very similar. And it cannot be doubted that the functional 

 differences are due entirely, or almost entirely, to differences 

 of anatomical connection, on the one hand with bones and 

 tendons, on the other with the nerve-cells of the spinal cord and 

 brain. The common properties in which all the skeletal muscles 

 agree are the subject-matter of the general physiology of striated 

 muscle. 



The cardiac muscle differs more, both in structure and in 

 function, from the skeletal muscles than these do among them- 

 selves ; the smooth muscle of the intestines and bloodvessels 

 still more. But every muscular fibre, striped or unstriped, 

 resembles every other muscular fibre more than it does a nerve- 

 fibre or a gland-cell or an epithelial scale. The properties 

 common to all muscle make up the general physiology of mus- 

 cular tissue. 



A nerve-fibre is at first sight very different from a muscular 

 fibre. It has diverged more widely from the primitive type 

 of undifferentiated protoplasm. It has lost the power of con- 

 traction, or contractility, but it retains, in common with the 

 muscle-fibre, susceptibility to stimulation, or excitability, the 

 capacity for growth, and to a limited extent the capacity for 

 reproduction. This inheritance of primitive properties, retained 

 alike by both tissues, is the basis of the general physiology of 

 muscle and nerve. 



The electrical organ of the Torpedo or the Malapterurus is 

 intermediate in some respects between muscle and nerve, and 

 has properties common to both. In the gland-cell the chemical 

 powers of native protoplasm have been specialized and de- 

 veloped. Contractility has been, in general, entirely lost ; 

 but excitability remains. The idea that certain common 

 endowments find expression in the action of muscle, nerve, 

 electrical organ, gland, etc., in the midst of all their apparent 

 differences, is the basis of the general physiology of the excitable 

 tissues. 



Amoeboid movement is the most primitive, the least elabo- 

 rated form of contraction. An amoeba may be seen under 

 the microscope to send out pseudopodia, or processes, of its 

 substance, and to retract them, and it is able by such movements 



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