48 HUMAN PHYSIOLOGY. 



GENERAL PHYSIOLOGY OF MUSCLE 

 TISSUE. 



The muscle tissue, which closely invests the bones of the body, 

 and which is familiar to all as the flesh of animals, is the immediate 

 cause of the active movements of the body. This tissue is grouped 

 in masses of varying size and shape, which are technically known as 

 muscles. The majority of the muscles of the body are connected 

 with the bones of the skeleton in such a manner that, by an altera- 

 tion in their form, they can change not only the position of the bones 

 with reference to one another, but can also change the individual's 

 relation to surrounding objects. They are, therefore, the active 

 organs of both motion and locomotion, in contradistinction to the 

 bones and joints, which are but passive agents in the performance 

 of the corresponding movements. In addition to the muscle masses 

 which are attached to the skeleton, there are also other collections 

 of muscle tissue surrounding cavities such as the stomach, intestine, 

 blood-vessels, etc., which impart to their walls motility, and so influ- 

 ence the passage of a material through them. 



Muscles produce movement of the structures to which they are 

 attached by the property with which they are endowed of changing 

 their shape, shortening or contracting under the influence of a stimu- 

 lus transmitted to them from the nervous system. Muscles are there- 

 fore divided into : 



1. Voluntary muscles, comprising those whose activity is called forth 

 by stimuli of the nerves as the result of an act or effort of volition. 



2. Involuntary muscles, comprising those whose activity is entirely 

 independent of the volition. 



The voluntary muscles are also known from their attachment to 

 the skeleton as skeletal, and from their microscopic appearance as 

 striped muscles. The involuntary muscles, from their relation to 

 the viscera of the body, are known also as visceral, and from their 

 microscopic appearance as plain or smooth muscles. 



General Structure of Muscles. All skeletal muscles consist of 

 a central fleshy portion, the body or belly, which is provided at either 

 extremity with a tendon in the form of a cord or membrane by which 

 it is attached to the bones. The body is the contractile region, the 

 source of activity ; the tendon is a passive region, and merely trans- 

 mits the activity to the bones. 



