THE PHYSIOLOGY OF MOVEMENT. 39 



The internal movements are exhibited by the viscera, the vascular appa- 

 ratus, and by glands, and, though less obvious, are no less characteristic. 



The viscera, by virtue of the presence of non-striated muscle-fibers in 

 their walls, are capable of changing their caliber from moment to moment 

 either in the way of an increase or decrease and thus regulate and control 

 the passage of their contents through them. 



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

 engaged in the distribution of blood and nutritive material throughout the 

 body. The heart drives the blood through the vessels in opposition to the 

 friction presented by their walls, while the vessels themselves and especially 

 the arteries, by virtue of 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. 



When these various organs are applied to the overcoming of various 

 resistances or forces, as they are in the performance of their functions, it can 

 also be said that they too are doing work. The cooperation 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. 



The various tissues of the body, mentioned in foregoing 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 

 excited to activity by agencies other than the nerve impulse are nevertheless 

 augmented or inhibited in their activity from moment to moment by nerve 

 impulses. 



It is evident therefore that the activities of the organs and tissues which 

 are engaged in promoting the work of the body are excited to action and 

 controlled by the nerve tissue, a fact which presupposes an anatomic con- 

 nection between them. 



For an understanding of the mode of excitation of the motor organs and 

 the manner in which they cooperate in the performance of any given move- 

 ment, a brief preliminary account of the general arrangement and mode of 

 action of the nerve tissue will be found helpful. 



The General Relation of the Nerve Tissue to the Motor Organs. 

 The nerve tissue is arranged partly in masses contained within the cavities 

 of the head and spinal column (the encephalon or brain and spinal cord), 

 forming the central organs of the nerve system, and partly in the form of 

 cords or nerves, (the cranial and spinal nerves), forming the peripheral organs 

 of the nerve system. The latter connect the former not only with muscles, 



