600 THE HUMAN BODY. 



which pass directly from the dorsal spinal roots to the brain 

 without intervention of the gray matter of the cord ; but some 

 of its fibres pass into the gray matter of the cord before 

 reaching the medulla oblongata. Finally, the tracts which 

 show no special ascending or descending degenerations are 

 mainly made of longitudinal commissural fibres uniting differ- 

 ent regions of the gray matter of the cord. 



The Spinal Cord as a Reflex Centre. In order to explain 

 physiological facts we must assume in addition to the special 

 paths of union between parts of the gray matter of the cord 

 afforded by certain fibres of the white columns, first, that a 

 nervous impulse entering the gray network at any point may, 

 under certain conditions, travel all through it, and give rise 

 to efferent impulses emerging at any level; and, on the other 

 hand, that there are certain lines or paths of easiest propa-. 

 gation between different points in this network, which the 

 impulses keep to under ordinary conditions. 



When a frog is decapitated it lies down squat on its belly 

 instead of assuming the more erect position of the uninjured 

 animal; its respiratory movements cease (their centre being 

 removed with the medulla) ; the hind legs at first remain 

 sprawled out in any position into which they may happen to 

 fall, but after a time are drawn up into their usual position, 

 with the hip and knee-joints flexed : having made this move- 

 ment the animal, if protected from external stimuli, makes 110 

 other by its skeletal muscles ; it has lost all spontaneity, and 

 only stirs under the influence of immediate excitation. Nev- 

 ertheless the heart goes on beating for hours; the muscles 

 and nerves, when examined, are found to still have all their 

 usual physiological properties; and, by suitable irritation, the 

 animal can be made to execute a great variety of complex 

 movements. But it is no longer a creature with a will, doing 

 tilings which v/e cannot predict ; it is an instrument which 

 can be played upon, giving different responses to different 

 stimuli (as different notes are produced when different keys 

 of a piano are struck), and always the same reaction to the 

 same stimulus; so that we can say beforehand what will hap- 

 pen when we touch it. Such actions are called reflex or excito- 

 molor and fall into two groups : (1) orderly or purpose-like 

 reflexes, which are correlated to the stimulus and are often 

 defensive, tending, for instance, to remove an irritated part 

 from the irritating object ; (2) disorderly or convulsive reflexes. 



