548 THE POPULAR SCIENCE MONTHLY. 



the Will can to a certain extent control, but which are not in such im- 

 mediate relation to it as is the brain. You all know that there passes 

 down our backbone a cord which is commonly called the " Spinal Mar- 

 row." Now, this spinal marrow gives off a pair of nerves at every 

 division of the backbone ; and these nerves are double in function one 

 set of fibres conveying impressions from the surface to the spinal cord, 

 the other motor impulses from the spinal cord to the muscles. Now, it 

 used to be considered that this Spinal Cord (I use the term spinal cord, 

 which is the same as spinal marrow, because it is just as intelligible 

 and more correct) was a mere bundle of nerves proceeding from the 

 brain ; but we have long known that this is not the case, that the spinal 

 cord is really a nervous centre in itself, and that if there were no brain 

 at all the spinal cord would still do a great deal. For example, there 

 have been infants born without a brain, yet these infants have breathed, 

 have cried, have sucked, and this in virtue of the separate existence and 

 the independent action of this spinal cord. Let us analyze one or two 

 of these actions. We will take the act of Sucking as the best example, 

 because experiments have been made upon young puppies, by taking 

 out the brain, and then trying whether they would suck ; and it was 

 found that putting between the lips the finger moistened with milk or 

 with sugar and water, produced a distinct act of suction, just as when 

 an infant is nursed. Now, how is this ? It is what we call a " reflex 

 action." I shall have a good deal to say of reflex action higher up in 

 the nervous system, and therefore I must explain precisely what we 

 mean by that term. It is just this : There is a certain part of the spi- 

 nal cord, at the top of the neck, which is what we call a ganglion, that 

 is, a centre of nervous power : in fact, the whole of the spinal cord is a 

 series of such ganglia ; but this ganglion at the top of the neck is the 

 one which is the centre of the actions which are concerned in the act 

 of sucking. Now, this act of sucking is rather a complicated one, it in- 

 volves the action of a great many muscles put into conjoint and har- 

 monious contraction. We will say, then, that here is a nervous centre. 

 [Dr. Carpenter made a sketch upon the black-board.] These are nerves 

 coming to it, branches from the lips ; and these another set going to 

 the muscles concerned in the movement of sucking from it. Thus, by 

 the conveyance to the ganglionic centre of the impression made on the 

 lips, a complicated action is excited, requiring the combination of a 

 number of separate muscular movements. We will take another ex- 

 ample the act of Coughing. You feel a tickling in your throat, and 

 you feel an impulse to cough which you cannot resist ; and this may 

 take place not only when you are awake and feel the impulse, but when 

 you are asleep and do not feel it. You will often find persons coughing 

 violently in sleep, without waking or showing any sign of consciousness. 

 Here, again, the stimulus, as we call it, produced by some irritation in 

 the throat, gives rise to a change in the nerves going toward the gan- 

 glionic centre, which produces the excitement of an action in that 



