488 ELEMENTARY PHYSIOLOGY less. 



Using the expression reflex action in a rather wider and 

 more general sense we may here again draw attention to 

 the importance of these actions t(j the working and wel- 

 fare of the body as regards the relationships of its internal 

 mechanisms. Thus we have seen that certain parts of the 

 spinal bulb, or medulla, which for our present purpose 

 may be regarded as part of the spinal cord, are connected 

 with the heart (cardio-inhibitory centre), blood-vessels 

 (vaso-motor centre), and respiratory muscles (respiratory 

 centx'e) in such a way that impulses arising in outlying 

 parts of the body, lead reflexly to such modified activity 

 of each of the above systems, as may from time to time be 

 necessary. (See pages 7o, 70, 158). 



Reflex action is a property of the central nervous 

 system which is not confined to the spinal cord alone, or 

 to the spinal bulb to which we have just extended it, 

 but is also a maiked characteristic of the varied activities 

 of the brain. But to this point we shall return later 

 on. 



Tbe Paths of Conduction of Afferent and Efferent 

 Impulses along the Spinal cord. The spinal cord has 

 a further most important function beyond reflex action, 

 namely that of ti-ansmitting nervous impulses, as a great 

 mixed motor and sensory nerve leading from the brain, 

 between the brain and the various organs, such as the 

 muscles and the skin, with which the spinal nerves are 

 connected. When we move a foot, certain nervous 

 impulses, starting in some part of the cerebi'al hemisphei'es, 

 pass down along the whole length of the spinal cord as 

 far as the roots of the spinal nerves going to the legs, and 

 issuing along the fibres of the anterior bundles of these 

 roots find their way to the muscles which move the foot. 

 Similarly, when the sole of the foot is touched, afferent 

 impulses travel in the reverse way upward along the 

 spinal cord to the brain. And the question arises, in 

 what manner do these efierent and afferent impulses travel 

 along the spinal cord ? 



