9i8 THE CENTRAL NERVOUS SYSTEM 



is obviously in accord with the interpretation of the tonus as a 

 postural reflex that in the mammal the extensor limb muscles should 

 exhibit the phenomenon and the flexors not, while in the frog it is 

 the reverse. For it is the extensors which are concerned in the re- 

 flex standing posture so characteristic of the mammal, while the 

 flexors of the hind limbs are especially concerned in the squatting 

 posture so characteristic of the frog. 



The postural reflexes differ in no essential way from the reflexes 

 previously studied, except that they are directed to the maintenance 

 of posture and not to the production of movement. Reflex postural 

 tonus has also been demonstrated for other groups of skeletal muscles, 

 of which the most important are the neck muscles concerned in the 

 position of the head. The reflexes in this instance originate not only 

 through the afferent fibres of the muscles themselves, but also from the 

 otic labyrinth, in discussing the functions of which the subject must be 

 returned to. But it may still be mentioned here, and it shows how 

 intricate is the correlation of the postural reflexes to one another, 

 that not only does the labyrinth influence the postural contraction of 

 the extensors of the limbs directly i.e., through afferent fibres from 

 the labyrinth itself, but also indirectly through the changes produced 

 in the posture of the head and neck i.e., through the afferent fibres 

 of the neck muscles (Magnus and de Kleijn). Even when the head of 

 a normal rabbit is passively moved so that its position relatively to the 

 body is altered, the postural tonus of the muscles of the extremities 

 and even of some of the muscles of the trunk, especially those connected 

 with the lumbar vertebrae, is definitely affected. 



A remarkable property of a muscle which exhibits postural reflex 

 tonus is that its tension remains approximately unaltered, even when 

 its length is greatly changed by placing the joint on which it acts in 

 different postures. Within a wide range it is able to counterbalance 

 just the same extending force whatever the length of the active muscle 

 may be (Practical Exercises, p. 1000). This property is dependent upon 

 the integrity of the reflex arcs. A muscle which is not under the 

 influence of its nerve centre reveals no trace of this peculiarity, which, 

 however, is possessed by the smooth muscle of viscera e.g., the urinary 

 bladder and the stomach. It has been shown that these hollow organs 

 can hold very different quantities of liquid without any great change 

 of tension. With a larger volume of contents, indeed, the tension 

 may be even less than with a smaller volume. It is obvious that this 

 property must play an important part in regulating the pressure 

 within such viscera, so that in the case of the bladder e.g., the pres- 

 sure may not rise prematurely to the point at which the desire to 

 micturate is aroused. The adjustment of the tonus of these smooth 

 muscles does not depend altogether upon the central nervous system, for 

 it is still exhibited, although not so amply, by the excised living stomach. 



Other characteristics of the reflex postural contraction are the long 

 periods for which it usually endures without apparent fatigue, and. 

 what is probably associated with this relative unfatiguability, the 

 slight expenditure of energy by Avhich it is maintained (Roaf) . 



It is probable that the tone of such visceral muscles as the sphincters 

 of the anus and bladder has a reflex element analogous to postural 

 contraction, and possible that the same is true of the tone of the smooth 

 muscular fibres of the bloodvessels on which the maintenance of the 

 mean blood-pressure so largely depends (p. 185). 



