THE SPINAL CORD.— GENERAL. 469 



which from the law of habit come to act together when a 

 stimulus is applied. Naturally those that make for the welfare 

 of the animal are such as are most used under the influence of 

 the intelligence of the animal — i. e., of the domination of the 

 higher cerebral centers, so that when the latter are removed it 

 is but natural that the old mechanisms should be still employed. 

 Moreover, the reflex movements are not always beneficial, as 

 when a decapitated snake coils itself around a heated iron 

 under reflex influence, which is readily enough understood if 

 we remember the habit of coiling around objects, and what 

 this involves— viz., organized tendencies. 



Inhibition of Keflexes. — It can be shown in the case of a frog 

 that still retains its optic lobes and the parts of the brain pos- 

 terior to them that, when these are stimulated at the same time 

 as the leg, the reflex, if it occurs at all, is greatly delayed. 



On the other hand, in the case of dogs, from which a part 

 of the cerebral cortex has been removed, the reflexes are much 

 more prominent than before. Experience teaches us that the 

 acts of defecation, micturition, erection of the penis, and many 

 others, are susceptible of arrest or may be prevented entirely 

 when the usual stimuli are still active, by emotions, etc. 



These and numerous other facts tend to show that the higher 

 centers of the brain can control the lower ; and it is not to be 

 doubted that pure reflexes during the waking hours of the 

 higher animals, and especially of man, are much less numerous 

 than among the lower vertebrates. The cord is the servant of 

 the brain, and a faithful and obedient one, except in cases of 

 disease, to some forms of which we have already referred. 



THE SFINAL CORD AS A CONDUCTOR OF IMPULSES. 



It is to be carefully borne in mind now, and when studying 

 the brain, that a conducting path in the nervous centers is not 

 synonymous with conducting fibers. The cells themselves 

 and the neuroglia probably are also conductors. We shall 

 now endeavor to map out, as established by the method of 

 Flechsig, Waller, and others, the main fiber tracts of the spinal 

 cord. 



1. Antero-median Columns (columns of Turck). — These 

 probably decussate in the cervical region, where they are most 

 marked, constituting the direct or uncrossed pyramidal tract 

 and disappear in the lower dorsal region. 



