42 TEXT-BOOK OF PHYSIOLOGY 



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 Peripheral Organs. 

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



,sp.c 



FIG. 13. DIAGRAM SHOWING THE STRUCTURES INVOLVED IN THE PRODUCTION OF REFLEX 

 ACTIONS. (G. Bachmann.) r.s. Receptive surface; af.n. afferent nerve; e.c. emissive or motor 

 cells in the anterior horn of the gray matter of the spinal cord, sp.c.; ef.n. efferent nerves distributed 

 to responsive organs, e.g., directly to skeletal muscles, sk.-m., and indirectly through the inter- 

 mediation of sympathetic ganglia, sym.g., to blood-vessels, b.v., and to glands, g. The nerves 

 distributed to the walls of the viscera are not represented; v.r. and d.r. t ventral and dorsal roots 

 and spinal nerves. 



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, 

 glands, blood-vessels, and viscera, but with the skin, mucous membranes, 

 etc., as well. 



(The relation of the nerve-tissue to the skeletal muscles, to glands, to 

 blood-vessels, and viscera are shown in Figs. 12, 13.) 



The Spinal Cord. The spinal cord is more especially the seat of origin 

 of the nerve energy that immediately excites and controls the activity of 

 the motor organs, and a knowledge of its structure, of its relations to these 

 organs, and the manner in which it is, in turn, excited to activity is necessary 

 to an understanding of the problem of movement. 



