SPECIAL PHYSIOLOGY. 47 



shock, cold, tickling, and other excitants of the centripetal 

 nerves, will produce movements as well as secretions. 



Reflex Movements. The spinal cord can also produce 

 certain very complicated reflex movements with assistance 

 of the brain ; of this kind are the movements of defence that 

 are observed in those decapitated animals who may be sub- 

 jected to irritations (frogs or tritons). Most generally the 

 .movements of progression (walking, leaping, or swimming) 

 are made without the intervention of intellectual action ; 

 volition can be completely unconcerned in the process of 

 walking, and we ordinarily walk without knowing it, as we 

 might say. This phenomenon is the act and even the exclu- 

 sive act of the spinal cord. The brain is concerned only at 

 certain times, when, for instance, we desire to regulate the 

 speed either by retarding or hastening our step. 



From the moment it is admitted that all organic acts are 

 the result of a peripheral impression, all these acts have a 

 reflex character ; thus all organs present in the study of their 

 functions a series of reflex acts in which we shall see the 

 spinal cord acting, not as auxiliary to the brain, but as a true 

 centre, which in certain cases can act for itself alone. A few 

 examples of reflex acts will help us more clearly to under- 

 stand the method of the function of the nerve centres and of 

 the spinal cord in particular. 



Sneezing is a phenomenon provoked either by an excita- 

 tion brought to bear on the nasal mucous membrane or by a 

 sudden shock of the sun's rays on the membranes of the eye. 

 This peripheral irritation is transmitted by the trifacial nerve 

 to the Gasserian ganglion, whence it passes by a commissure 

 to an agglomeration of globules in the medulla oblongata or 

 in the protuberance ; from this point, by a series of numerous 

 reflex and complicated acts, it is transformed by the media- 

 tion of the spinal cord into a centrifugal excitation which 

 radiates outwards by means of the spinal nerves to the expir- 

 atory muscles. 



The respiratory movement depends on the spinal cord. 

 This presides over the regularity of respiration ; in order to 

 set up this reflex phenomenon the sensitive surfaces of the 

 trachea and of the pulmonary vesicles (air cells) must receive 

 an impression from the introduction of external air, or by air 

 vitiated and loaded with carbonic acid following the pul- 

 monary gaseous exchanges. 



The movements of the heart are the result of analogous 

 mechanism ; these are possible only when the internal surface 



