226 ANIMAL BIOLOGY. [Parti. 



region on either side of the vertebral column, the ganglia lying 

 on the heads of the ribs. Traced forwards into the neck, it is 

 seen as a flat nerve, on which are developed a posterior cervical 

 ganglion a little above the subclavian artery, and an anterior 

 cervical ganglion just anterior to the larynx. Traced backwards, 

 it gives off, about the level of the ninth thoracic ganglion, a 

 large splanchnic nerve, which is much more conspicuous than 

 the slender continuation of the chain. It enters a group of 

 greyish pink transparent ganglia, constituting the solar plexus, 

 of which the coeliac ganglion and the mesenteric ganglion, one 

 on either side of the anterior mesenteric artery, are the most 

 important. 



In the pigeon there is a cervical ganglion near the brachial 

 plexus. Anterior to this the sympathetic lies in the verte- 

 brarterial canal. The two sympathetic chains unite posteriorly 

 in a ganglion impar near the end of the caudal vertebrae. 



The Functions of the Nervous System, The function of the 

 nervous system, as a whole, is to form a centralised means of 

 intercommunication between the various parts of the organism, 

 and thus to convert an aggregate of organs into a living unit. 

 Closely connected with the nervous system are end-organs, (a) 

 for receiving impressions from without, as in the organs of 

 special sense ; and (b) for delivering impulses, as in the muscle- ' 

 plates. Space need not here be occupied in repeating what has 

 been said in Chapter IY. on reflex action ; nor is it within the 

 scope of this volume to enter further into the nervous 

 mechanism of responsive and original action. The following 

 brief notes must here suffice : 



1. Afferent end-organs (e.g. special senses) receive impressions 

 from without. 



2. Efferent end-organs (e.g. muscle-plates) deliver stimuli to 

 muscles or glands and other parts. 



3. Afferent and efferent nerve-fibres transmit these impres- 

 sions, the former towards, the latter awa,y from, the chief nerve- 

 centres. 



