THE SPINAL COKD.— GENERAL. 479 



was not momentarily required for the act of walking and all else 

 that is involved in the ahove behavior. It suggests that certain 

 nervous and muscular connections have been formed, function- 

 ally at least. Plainly, then, we should not expect each indi- 

 vidual man's spinal cord to be the same, but that the series of 

 mechanisms of which every spinal cord is made up should differ 

 with experience ; and if this holds for individuals, how much 

 more must it be true of different groups of animals, the habits 

 of which differ so widely. 



All the facts go to show that the cord is made up of nervous 

 mechanisms — if we may so speak — which are naturally associ- 

 ated, both structurally and functionally, with certain nerves 

 and muscles; these, like the paths which impulses take to and 

 from the brain, though usual, are not absolutely fixed, though 

 more so as reflex than conducting paths, while they are con- 

 stantly liable to be modified in. action by the condition of 

 neighboring groups of mechanisms, etc. 



We have said less about the gray matter of the cord as a 

 conductor than its importance perhaps deserves. It is believed 

 by many that impulses which give rise to sensations of pain 

 always travel by the gray matter ; and there is not a little evi- 

 dence to show that, when none of the white columns are avail- 

 able, owing to operative procedure, disease, or other disabling 

 cause, the gray matter will conduct impulses that usually pro- 

 ceed by other tracts. 



Synoptical. — The spinal cord is composed of large ganglionic 

 nerve-cells, fibers, and connecting neuroglia. Functionally it 

 is a conductor, the seat of certain automatic centers and of 

 reflex mechanisms. Probably in every case the one function is 

 to a certain extent associated with the other — i. e., when the 

 cord acts reflex] y it is also a conductor, and the cells concerned 

 are so readily excited to certain discharges of nervous energy 

 that automaticity is suggested, and so in other instances: thus, 

 in the case of automaticity, reflex influence or afferent impulses 

 are with difliculty entirely excluded from consideration. 



The great majority of conducting fibers seem to cross either 

 in the cord itself or in the medulla oblongata. The conducting 

 paths that have been shown by pathological and clinical inves- 

 tigation to be best marked out in the spinal cord are those for 

 voluntary motor impulses. So far as the functions of the 

 human organ are concerned, clinical and pathological facts 

 have thrown the greatest amount of direct light on the subject; 



