164 NERVOUS SYSTEM OF VERTEBRATES. 



to the direct cerebellar tract and its nucleus in man, but it is very 

 desirable that the structures should be worked out in inter- 

 mediate classes, especially reptiles and lower mammals. 



Since the visceral centers are most highly developed in those 

 forms in which the gustatory organs are most numerous, the further 

 description of the secondary visceral nuclei will be given in the 

 following section. 



Special Visceral or Gustatory System. 



In fishes, as already stated (p. 22), the gustatory organs have 

 a much wider distribution than in higher vertebrates. As far as 

 is at present known the distribution of taste organs in the different 

 classes of vertebrates is as follows. In cyclostome larval forms 

 they have been found only in the pharynx, on the inner surface of 

 the branchial arches. In adults they are also irregularly distrib- 

 uted over the surface of the head and branchial region of the 

 body. Whether they occur in the trunk and tail region has not 

 been ascertained. In ganoids they are present in the mouth 

 and pharynx and in considerable numbers over the surface of 

 the head. In embryos of bony fishes they are found in the pharynx, 

 oesophagus and mouth. In adults they reach their greatest develop- 

 ment both as to number and distribution, being found also on 

 the surface of the head and in many forms on the fins and practi- 

 cally over the whole body. In amphibia they are found on the 

 tongue and mucosa of the mouth, being especially numerous on 

 the papillae of the tongue. The so-called multicellular glands 

 in the roof of the pharynx of tadpoles are probably taste organs. 

 In man they are found in small numbers on the general surface 

 of the tongue, more numerous along the sides of the tongue, and 

 most numerous on and around the circumvallate papillae and on 

 a region at the back of the tongue corresponding to the papillae 

 foliatae of some mammals. They are also numerous on the 

 anterior surface of the soft palate and on the posterior surface 

 of the epiglottis. 



The following facts show that the taste organs have their origin 

 in the entoderm. (i) They arise in the lining of the pharynx in 

 Ammocoetes, and are not found in the skin during larval life. 



