300 ESSENTIALS OF ZOOLOGY 



and viscera. The thoracic ganglia give rise to an important 

 nerve, the great splanchnic, which pierces the diaphragm and 

 meets branches coming from the gastric branch of the vagus 

 to form a ganglion situated in the mesentery between the 

 coeliac and anterior mesenteric arteries. This is called the 

 coeliac ganglion. The coeliac ganglion is connected with the 

 anterior mesenteric ganglion placed just behind the artery of 

 that name, and again with a posterior mesenteric ganglion 

 lying just in front of the posterior mesenteric artery. The 

 involuntary muscles of the viscera, the adrenals, and other 

 organs are supplied by the innumerable branching fibres of a 

 system which unites cranial and spinal nerves with an efficient 

 distributing system (fig. 139). 



The complex body of the mammal is thus under the control 

 of a nervous system which brings every part into relationship 

 with the rest, and the system is perfected by chemical integra- 

 tion of endocrines. In addition, the animal is brought into 

 relationship with its environment by afferent nerves which are 

 distributed to the skin and to special organs which have been 

 formed from the skin. These are the special sense organs. 



Sense Organs. The olfactory organ is situated in the 

 internal part of the nose. The epithelium is spread and 

 extended over the labyrinth provided by the foldings of the 

 ethmoturbinal. It consists of columnar cells supporting the 

 sense cells. The sense cells are spindle-shaped, the free end 

 terminating in a process lying in the mucus which covers the 

 surface, and the other continued as the olfactory fibril to the 

 olfactory lobe of the brain. It acts as a receptor of chemical 

 products carried by the air, and is thus closely associated with 

 taste. 



The eye, broadly speaking, is essentially that of the fish and 

 of the amphibian, but it is refined in structure, and it is brought 

 into important relationships with other centres in the nerve 

 endings in the cerebrum. Anatomically, it has the structure 

 which has already been described (p. 179). The retina is 

 the essential receptor organ, and is formed of rods and cones 

 directed to the back of the eye. On the other side that is to 

 say, between them and the rays of light they are related to 

 nerve cells, and through the cells to nerves which run together 



