THE PHYSIOLOGY OF MOVEMENT. 41 



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 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 excited to activity is necessary to an understanding of the prob- 

 lem of movement. 



The spinal cord is narrow and cylindric in shape and occupies the spinal 

 canal from the level of the first vertebra as far down as the second or third 



,sp.c. 



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

 ACTIONS. (G. Bachman.) 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, vr. and d.r., ventral and dorsal roots 

 and spinal nerves. 



lumbar vertebra. It presents both on its ventral and dorsal surfaces a 

 deep longitudinal fissure which partly divide the cord into halves, a right 

 and a left. To each side of the cord there is attached thirty-one nerves, 

 which as they pass out through foramnia in the walls of the spinal column are 

 termed spinal nerves. Each spinal nerve is connected with the spinal cord 

 by two roots, termed from their relation to the ventral and dorsal surfaces, 

 the ventral and dorsal roots. 



Experimental investigation has demonstrated that the ventral roots are 

 connected peripherally with the motor organs and transmit to them nerve 

 energy developed in the spinal cord; that the dorsal roots are connected 

 peripherally with the skin, mucous membranes, etc., and transmit nerve 

 energy developed in their terminations to the spinal cord. The ventral 

 and dorsal roots are therefore termed from their function, efferent and afferent 

 nerves respectively. 



A transverse section of the spinal cord shows that each half is composed 

 externally of white matter, and internally of gray matter. The gray matter 



