782 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



to these two groups various so-called sporadic ganglia are also to be 

 found in different parts of the body, whose functions, while in the main 

 of the same character as all nerve-cells wherever found, differ according 

 to the functions of the organs with which they are in connection ; they 

 will subsequently receive attention. 



The external ganglia, or the so-called nerve-corpuscles, vary in char- 

 acter according to the nature of the stimulus which it is their function 

 to receive. They may be distributed over the skin surface and are 

 fitted for recognition of tactile and thermic changes, or they may be 

 specialized for receiving special sensations ; in such cases they constitute 

 the organs of special sense. 



In addition to these terminals of nerves, which, it will be recog- 

 nized, are simply in connection with afferent nerves, another set of nerve- 

 corpuscles is in connection with the peripheral terminations of the motor 

 nerves and act as organs of distribution for motor impulses. In this 

 group fall the nerve-plates on the voluntary muscles and the ganglionic 

 cells in the walls of the intestinal tube. 



The central nervous ganglia, located in the cerebro-spinal axis, pos- 

 sess the power of developing, first, reflex action; second, automatism; 

 third, inhibition; fourth, augmentation; fifth, co-ordination. These will 

 be alluded to in detail. 



Nervous centres are capable of receiving impulses brought to them 

 through afferent nerves, multiplying them, and reflecting the impulses so 

 changed through an efferent nerve. A reflex action, therefore, requires 

 for its expression an afferent nerve, starting from some receptive sur- 

 face, some stimulus applied to that receptive surface, a nervous centre, 

 and an efferent nerve. 



1. Reflex Action. — Reflex actions may occur in a number of dif- 

 ferent ways. The impulse reaching the centre through a sensory nerve may 

 be reflected through a motor nerve and produce muscular contraction. 

 Such muscular movements, occurring reflexly as the result of stimuli, are 

 entirely involuntaiy and independent of the will. Such reflex actions are 

 almost innumerable and form an important part of the organic acts of a 

 living animal. As examples may be mentioned the involuntary weep- 

 ing which almost instantly follows the applications of a stimulus to the 

 conjunctiva, the movements of the limbs which occur on tickling the 

 soles of the feet daring sleep, movements of vomiting which occur when 

 the soft palate or pharynx are mechanically irritated, coughing following 

 irritation of the laryngeal or tracheal mucous membrane, and a large 

 number of other movements (Fig. 331). 



The spinal cord offers the best example of the production of reflex 

 motor actions, and, in fact, reflex action may be said to be the main 

 function of the spinal cord and its ganglionic cells may be regarded as 



