354 ZOOLOGY 



369. The Sympathetic System which is always distributed to the visceral 

 organs is made up of a series of connected ganglia in the dorsal part of the body 

 cavity. This system is in connection at various places with the central nervous 

 system. It serves to connect the internal organs more intimately, and is reflex in 

 its action. 



370. The Special Senses. — The sense organs represent 

 specialized terminations of the nerve fibres, or special epithelial 

 cells which have become associated with such fibres (Fig. 43). 

 From the very nature of the case they must be external. In the 

 case of higher animals, the more complicated sense-organs are 

 removed from the surface and are much modified, but the 

 essential sensory portion is similar in all, and they retain some 

 suitable connection with the outside. It is usually these ac- 

 cessory structures which transmit the stimuli to the nerves that 

 render the sense organ so complicated. 



371. The Skin Senses. — Scattered over the body of many 

 forms of animals are single cells, or groups of cells, or free nerve 

 endings, which are for the reception of contact and temperature 

 stimuli. These are not equally numerous or well developed in 

 all parts of the body. They are often especially developed in 



, connection with hairs. In the lower aquatic vertebrates, espe- 

 cially the fishes, groups of such sensory cells occur in pits or 

 longitudinal grooves along the sides. These are called the or- 

 gans of the lateral line. Their exact function is still in some 

 doubt. It is thought that they may possibly assist in the recog- 

 nition of slow vibrations in water. 



372. The Chemical Senses — Taste and Smell. — The chem- 

 ical senses involve close contact and a chemical union between 

 the substance to be perceived and the organ itself. For that 

 reason the substance must be capable of solution in the fluids 

 that moisten the surfaces. In vertebrates these organs are 

 located at the anterior end of the body and usually within special 

 pits or cavities. The taste organs are in the mouth, especially 

 on the tongue and soft palate. In some animals the sense is 

 poorly developed. The end organs of the sense of smell are 

 located in pits (nose), anterior or dorsal to the mouth, lined 

 with folds of the mucous epithelium. In most fishes these pits 



