40 TEXT-BOOK OF PHYSIOLOGY 



That a lever may be effective as an instrument for the accomplishment of 

 work it must not only be capable of moving around its fulcrum, but it must 

 at the same time be acted on by two opposing forces, one passive, the other 

 active. In the movements of the bony levers of the animal body, the 

 passive forces to be overcome are largely those connected with the environ- 

 ment, e.g., gravity, cohesion, friction, etc., the active forces by which 

 these are opposed and overcome through the mediation of the bony levers, 

 are found in the muscles attached to them. The muscles are therefore to 

 be regarded as the seat of those active energies that impart movement to 

 the levers. 



The Internal or Vasculo- and Viscero-muscle Movements and 

 Gland Activities. The internal movements are exhibited by the viscera, 

 the vascular apparatus and by glands, and, though less obvious, are no less 

 characteristic. 



The viscera undergo variations in size from moment to moment as a 

 result of the contraction and relaxation of the non-striated muscle fibers, 

 composing in part their walls and thus regulate the reception and discharge 

 of their contents. 



The vascular apparatus, and its adjunct, the lymph-vessel apparatus, is 

 engaged in the distribution of blood and nutritive material throughout the 

 body. The cavities of the heart are alternately increased and diminished 

 in size by the alternate relaxation and contraction of the muscle-fibers 

 composing the walls of the heart. During the relaxation the cavities fill 

 with blood and during the contraction the blood is driven through the vessels 

 in opposition to the friction presented by their walls; while the vessels them- 

 selves and especially the arteries, by virtue of the presence of elastic fibers 

 and the non-striated muscle-fibers in their walls, increase and decrease in 

 caliber from moment to moment and thus regulate the amount of blood 

 flowing through them in accordance with the physiologic needs of the organ 

 to which they are distributed. 



The glands and more especially their epithelial investments are the seat 

 of certain molecular movements the result of which is the production and 

 discharge of a secretion destined to play a more or less important part in the 

 maintenance of the activities of the body. 



In the performance of their functions these organs also meet resistances, 

 e.g., cohesion, friction, elasticity, etc., and when they are applied to the 

 overcoming of these resistances or forces, as they are in the performance of 

 their functions, it can also be said that they too are doing work. The co- 

 operation of external and internal organs is necessary, however, not only 

 for the maintenance of the life of the animal but also for the accomplishment 

 of external work. 



Tissue Stimuli. The various tissues of the body, mentioned in fore- 

 going paragraphs, though irritable do not possess spontaneity of action, 

 but require for the manifestation of their characteristic forms of activity 

 the application of a stimulus. 



Thus the skeletal muscles and glands though capable of being excited 

 to activity by various artificial stimuli, require for the exhibition of their 

 normal activity the arrival of the physiologic stimulus, the nerve impulse, 

 developed in and transmitted to them by the nerve-tissue. 



The visceral and vascular muscles though apparently capable of being 



