578 FUNCTIONS OF THE CEREBRO-SPINAL NERVOUS SYSTEM. 



In DO case is this adaptiveness more remarkable, than in some of those 

 actions, which are not only performed without any effort of the will, but 

 which the will cannot imitate. This is the case, for example, with the act of 

 Deglutition ( 87, 88), the muscles concerned in which cannot be thrown 

 into contraction by a voluntary impulse, being stimulated only by impres- 

 sions conveyed from the mucous surface of the fauces to the Medulla Oblon- 

 gata, and thence reflected along the motor nerves. No one can swallow, 

 without producing an impression of some kind upon this surface, to which 

 the muscular movements will immediately respond. Now it is impossible to 

 conceive any movements more perfectly adapted to a given purpose, than 

 are those of the parts in question ; and yet they are independent, not only 

 of volition but of sensation, being still performed in cases in which con- 

 sciousness is completely suspended or entirely absent. The act of Sucking 

 in the infant, again, is one in which a number of muscles are called into 

 combined contraction, in a manner which shows a complete adaptation to a 

 given purpose; and yet it is impossible to suppose this adaptation to be pur- 

 posive on the part of the infant itself; more especially as it is shown both 

 by the occurrence of monstrosities, and by experiments made with this 

 object, that no part of the Cranio-spinal axis above the Medulla Oblongata 

 is necessary to it. And in the acts of Coughing and Sneezing ( 303), we 

 have additional examples of the most adaptive movements, executed by a 

 marvellous combination of separate muscular actions, with the obvious pur- 

 pose of removing sources of irritation from the air-passages; and yet we 

 know by personal experience, that this combination is not made with any 

 design of our own. 



469. The activity of the nervous ganglia and of the nervous cords is alike 

 called into action by the application of stimuli. These vary in their nature, 

 and whilst some, as electrical currents, chemical agents, and mechanical 

 pressure, can excite all nerves to action, others, like the vibrations of light 

 and sound, odorous emanations, and heat, appear to be only capable of ex- 

 citing nerves whose intimate structure especially adapts them for responding 

 to the impressions made by these delicate yet active agents. Although the 

 conducting power of the Nerves for electrical currents is many thousand 

 times less than metallic threads, electricity appears to be a stimulus that very 

 readily affects them, whatever may be their function ; exciting not only the 

 various sensory nerves, but being also the most powerful agent with which 

 we are acquainted for inducing the contraction of muscles when applied to 

 the motor nerves that supply them. The effects of electricity upon nerves 

 differ remarkably according to whether a continuous or an induced current 

 is employed. A continuous current passed across a nerve at right angles 

 occasions no contraction ; but if it be directed up or down the nerve, con- 

 traction commonly occurs at the moment of closing and opening the current, 

 though none during its steady and uniform passage. With an induced or 

 interrupted current, on the contrary, the nerve being kept in a constant state 

 of excitation, the muscles supplied by it pass into a state of permanent or 

 tetanic contraction. With a continuous current, contraction is observed to 

 take place, not only at the moment of closing and opening, but also when 

 any sudden change, either of increase or decrease, occurs in the intensity of 

 the transmitted current. The observation of these phenomena has led to 

 the establishment of a law of nerve physiology, which was originally laid 

 down by I)u Bois-Reyinond, that the excitation of a nerve is effected not so 

 much by the absolute amount or intensity of the stimulus that is applied to 

 it, but by the modification of this amount from one moment to another. In 

 other words, the excitation of any nerve caused by a stimulus is greater the 

 more rapidly the resulting changes are produced, or the greater they are in 



