448 PHYSIOLOGY 



immediate result of stimulation of the other system, the ' exterocep- 

 tive,' which is distributed over the surface of the body. Since it is 

 excited by the movement of the muscles themselves, i.e. by the first 

 result of the reaction to external stimulus, it serves as a governing 

 mechanism to regulate the extent of each motor discharge. Its 

 excitation not only prevents over-action of the muscles, but may 

 evoke a compensatory reflex in an opposite direction to the reflex 

 immediately excited from the skin. A marked feature of this system 

 is its tendency to continued or tonic activity. The steady slight 

 contraction which is observable in most skeletal muscles, and is 

 spoken of as their ' tone,' is quite independent of the surface sensi- 

 bility and depends entirely on the proprioceptive system of the 

 muscles and their accessory structures. 



In the decerebrate animal the rigidity of a limb disappears at 

 once after section of its afferent roots, though it is unaltered by 

 division of the main skin nerves. This tonus does not affect all 

 muscles to an equal degree. In every limb there is a predominance of 

 tonus in certain muscles above others, so that the result on the whole 

 limb is an attitude or posture which is typical of the limb or the 

 animal. Thus the spinal frog takes up an attitude which is very 

 different from that which would be impressed on it by gravity in the 

 absence of muscular activity. If one of its hind limbs be extended 

 gently, it soon draws it up to reproduce the same crouching position. 

 The posture of the limb is therefore a result of afferent impressions 

 continually ascending its proprioceptive nerves and exciting a tonic 

 activity which predominates in certain definite muscles. This posture, 

 as carried out by the spinal cord, is a segmental response. It deter- 

 mines the relation of the limb to the trunk, and to a less extent of 

 the fore limbs to one another. It is not concerned with the relation 

 of the animal as a whole to its environment, and only to a slight 

 extent with the maintenance of equilibrium in the presence of the 

 continually acting force of gravity. 



In the evolution of the nervous system there has been a continual 

 subordination of the hinder parts to the head end, in consequence 

 of the development at this end of the all-important distance receptors, 

 the impulses from which take a predominating part in determining 

 the reactions of the body as a whole. In fact the subordination 

 of one part of the central nervous system to another is in direct 

 relation to the importance of the afferent impulses arriving at each 

 portion of the system. Thus the vaso-motor centres segmentally 

 distributed throughout the spinal cord are subject to the vaso-motor 

 centre in the medulla, which is developed at the point of entry of 

 the vagus nerves, i.e. the chief afferent nerves from the heart and 

 large blood-vessels. The collections of grey matter presiding over the 



