398 PHYSIOLOGY 



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

 traction, or ' tone,' which is observable in most skeletal muscles, is inde- 

 pendent of the surface sensibility 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, 

 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 determines the relation of the 

 limb to the trunk, and to a less extent of the four 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 

 segmental reactions of the intercostal muscles are entirely subordinated to 

 the grey matter in the medulla around the entry of the vagus fibres from 

 the lungs. 



This subordination of the hinder to the anterior sense-organs is paralleled 

 in the case of the proprioceptive system. Entering the hind-brain at the 

 upper border of the medulla is the eighth nerve, composed of two parts which 

 differ widely in functions, viz. the cochlear division and the vestibular 

 division. The former is entirely concerned with the reception of sound 

 waves, and 'is therefore the auditory nerve. The vestibular nerve, which is 

 distributed to the rest of the membranous labyrinth, must be assigned to the 

 proprioceptive system. The labyrinth is practically a double organ. The 

 primitive auditory sac arises as a simple involution of the surface. In the 

 course of development the front part is modified to form the canal of the 

 cochlea, which is set apart entirely for the reception of sound. From the 



