300 REFLEX ACTION. 



and that alterations capable of producing a paralysis of sensibility, and 

 situated upon any point of a lateral half of the cerebro-spinal axis, al- 

 ways produce a paralysis of sensibility on the opposite half of the body, 

 and that there is no difference between the brain and the spinal marrow 

 in this respect. 



Thus constructed, the spinal cord, as we shall presently show from 

 Analoo- with ^ r * Carpenter, evidently agrees with the gangliated ventral 

 ventral cord cord of the articulata, each portion x of it from which a pair of 

 lata ' nerves is given off representing each ganglion of that ventral 

 cord, the difference in the two structures being, that in the spinal col- 

 umn the ganglia are commissured, so as to form, in appearance, one con- 

 tinuous mass, and agreeably to this view of its construction are the 

 circumstances under which its enlargements occur. In those animal 

 forms in which the entire trunk is concerned in locomotion, as in snakes 

 and eels, the cord is nearly cylindrical; but as soon as special members 

 for locomotion are developed, a corresponding increase of diameter is ob- 

 served. Thus, in birds, the ganglionic enlargement corresponds with the 

 region from which the nerves for the wings are given off; but in that tribe, 

 as in the ostrich, the mode of locomotion of which is by the legs rather 

 than by the wings, a corresponding posterior enlargement occurs. The 

 same observations may even be more distinctly made during metamorph- 

 oses ; thus, in frogs, while they are in the tadpole state the spinal cord is 

 cylindrical, but bulging ensues in it anteriorly and posteriorly as soon as 

 the anterior and posterior members are developed. 



The translation of impressions which have been brought along the 

 Reflex action centripetal fibres into motions, the exciting influence of which 

 of the cord. { s conveyed along the centrifugal fibres, includes what is un- 

 derstood as the reflex action of the spinal cord as developed by Dr. Hall. 

 Its essential condition is its independence of the agency of the brain, and 

 therefore unconscious nature. As general examples may be mentioned 

 the movements which occur in swallowing ; for after the food has been 

 carried by voluntary action into the fauces, its passage onward to the 

 stomach is perfectly involuntary. In like manner, the introduction of air 

 into the lungs in ordinary respiration is involuntary ; for though it may 

 be, to a certain extent, under the control of the will, yet that extent is 

 limited, a necessity for the motion presently arising, which soon becomes 

 uncontrollable. The action of the valvular arrangements at the cardiac 

 and pyloric orifices of the stomach, and the constant contraction of the 

 sphincter *ani, are farther illustrations. To these may be added those 

 impulsive movements which we instinctively make on the approach of 

 danger or in the act of falling, and perhaps, too, automatic walking, as 

 we go from place to place in a state of mental abstraction, paying no at- 

 tention to the course we take. 



