48 NERVOUS SYSTEM. 



of the heart is in direct contact with the blood. This contact 

 plays the part of a peripheral impression. If it were possible 

 to empty the heart completely of all the blood that it con- 

 tained, it would stop its pulsations until a few drops of blood 

 should be introduced, after which its movements would con- 

 tinue; however the nerve globules that preside over this 

 reflex act are situated in the thick walls of the heart (See 

 circulation). 



Walking is also, as we have already said, a reflex phenom- 

 enon ; its point of departure is the peripheral impression pro- 

 duced by the contact of the foot with the ground. The sole 

 of the foot is plentifully supplied with tactile apparatus. If 

 this peripheral impression be but imperfectly transmitted to 

 the nerve centre, reflex action has no longer any regularity. 

 Thus when the great sciatic nerve has been compressed in 

 certain postures, during the few moments it remains para- 

 lyzed (for sensation only) walking becomes impossible or at 

 least painful. 



There are other examples of reflex action fully as impor- 

 tant as the preceding, principal among which are the secre- 

 tions. It is generally admitted as a rule that previous to 

 every secretion a peripheral impression is transmitted to the 

 nerve centres and thence to the gland. The salivary secre- 

 tion is dependent' on the centripetal nerves of taste which 

 convey the impressions of taste to the medulla oblongata, 

 whence they are reflected by means of the centrifugal nerves 

 (facial) to the glands and their vessels. These centrifugal 

 nerves seem to act directly on the cells of the secretory organ, 

 independently of the vascular elements ; for if the circulation 

 in a gland be suppressed simultaneously with the excitation of 

 its functions, it affords to the surrounding tissues the mate- 

 rials no longer furnished by the blood and the gland continues 

 to secrete. The secretion of gastric juice might be cited as 

 an example of the reflex action of whose existence we are 

 unconscious; but in this connection is presented the peculiar 

 fact,, that the secretion must be provoked by a suitable exci- 

 tant, an aliment (we shall mention at another time that the 

 introduction of foreign bodies, small pebbles, in the stomach 

 provokes no secretion of true gastric juice, but of a mucus 

 possessing no digestive properties). 



'We have remarked that, in the eyes of the physiologist, 

 the spinal cord extends as far as the sella turcica. This view 

 is sustained by the study of reflex actions whose centre is in 

 the cranial portion of the cord ; there, as well as in the spinal 



