338 AN INTRODUCTION TO ZOOLOGY 



sends off a small twig to join the common trunk. The main trunk 

 on each side is connected anteriorly with the sphenopalatine ganglion 

 and communicates with the intra-cranial portions of the trigeminal, 

 glosso-pharyngeal and vagus. Extra-cranially it is connected also 

 with the hypoglossal. At the anterior end of the neck it lies close 

 to the trachea, slightly dorsal and mesial to the carotid artery, and 

 there enlarges to form an anterior cervical ganglion, partly in front of 

 the vagus ganglion. It passes backwards, and at the hinder end 

 of the neck just in front of the sub-clavian artery it enlarges to form 

 a median cervical ganglion. The chain is connected with a plexus 

 of sympathetic fibres in the heart, and then passes back into the 

 abdominal cavity, where it is related to three median ganglionic 

 enlargements. The first of these is the coeliac ganglion, lying 

 between the coeliac and anterior mesenteric arteries. The second 

 is the anterior mesenteric ganglion, situated just behind the similarly 

 named artery. Fibres run back from this to a much smaller 

 posterior mesentric ganglion lying in the mesentery just in front of 

 the artery of the same name. These three ganglions, particularly 

 the front two, are sometimes referred to as the solar plexus. 



The fibres from the sympathetic chain are distributed to all 

 the viscera and regulate the activity of the non-striate muscles so 

 that the system is sometimes spoken of as the involuntary nervous 

 system. 



Sense Organs. 



Little remains to be noted in regard to the sense organs, 

 since they have already been dealt with sufficiently in other forms, 

 and, indeed, the description of the eye given was based mainly 

 upon that of a mammal. 



The olfactory organ is very well developed and occupies the 

 large anterior nasal chamber. Although a large space it is almost 

 completely filled with the complicated turbinal bones, over all of 

 which spreads the typical olfactory neuro-epithelium. 



The details of the structure of the eye call for no further ex- 

 pansion, but it will be seen from the position that it occupies, well 

 sunk in the deep orbit at the side of the skull, that each eye looks 

 out to its own side, and that there is practically no overlapping of 

 the two fields of vision as in our own case. 



It is in the ear that we meet with the greatest advance over the 

 conditions in Rana. In the first place the tympanic membrane is 

 no longer upon the external surface of the skull, but has sunk down 

 a considerable way, and the wide canal leading from it to the 

 outside, i.e. the external auditory meatus, is enclosed in the tympanic 

 bone. On .the outside of the skull, in order to compensate for the loss 



