442 



A MANUAL OF VETERINARY PHYSIOLOGY 



effect over reflex actions. One very prominent feature of a 

 reflex act is its apparently purposeful character. An oppor- 

 tunity for studying this in detail will be given presently when 

 the ' reflex frog ' is considered. 



In the dog very characteristic reflex actions occur after division 

 of the cord, such as those of walking, running, scratching, mic- 

 turition, and defalcation ; and some of these will shortly be con- 



The parts within the dotted 



line lie within the grey 



„ »* matter of the nerve centre. 



Re, Re, Receptive surface, 

 in this case the skin, im- 

 pressions from which are 

 conducted by afferent 

 fibres, dd, ax, to a nerve 

 centre, pk', where out- 

 going impulses are gener- 

 ated ; these are conducted 

 by an efferent channel, 

 ax', to a discharging body, 

 Ef, in this case muscle. 



The figure is also employed 

 to illustrate conduction 

 and transmission of im- 

 pulses along a nerve arc. 

 The dotted line encircles 

 a nerve centre (in this case 

 the spinal cord), dd then 

 become dendrites on their 

 way to the perikaryon (see 

 note, p. 433). pk (in this 

 case the ganglion on the 

 dorsal root of the spinal 

 nerve), or if the peri- 

 karyon be short-circuited, 

 the impulse passes direct 

 to the axon, ax ; this Con- 

 stitutes the neuronic path, 

 sy, sy are synapses, by 

 which the impulse is con- 

 ducted to the next neu- 

 rone, through the medium 

 of dendrites, dd', dd', and 

 thence to pk', the cell-body 

 or perikaryon ; this consti- 

 tutes the synaptic path. 



Fig. 



132. — Diagram of a Reflex Arc 

 (Sherrington). 



sidered in detail, owing to their deep physiological importance. 

 The higher the animal scale is ascended, the less easy is it to 

 obtain evidence of free spinal reflexes — viz., reflexes which take 

 place without any guidance from the brain. This may perhaps 

 be due to a more constant influence exercised over them by the 

 brain. Nevertheless, animals as highly developed as the horse 

 and ox give the most undoubted evidence of reflex spinal acts. 

 If the spinal cord of either be severed behind the medulla, a cut 



