l88 NERVOUS SYSTEM OF VERTEBRATES. 



which the organs are constantly bathed. In both cases the nerve 

 impulse is probably aroused by the entrance into the sense cell 

 of a small modicum of the stimulating substance. Since there 

 is this similarity in the stimuli and in the purpose which the gusta- 

 tory and olfactory organs serve, the question whether there is 

 any morphological relation between the two becomes of con- 

 siderable interest. 



A direct comparison of the olfactory organ and the taste buds 

 is impossible, for the reason that the sense cells of the olfactory 

 organ themselves give rise to the fibers which carry their impulses 

 to the central nervous system, while the fibers which carry taste 

 impulses arise from gangHon cells situated in the cranial nerve 

 gangha. The olfactory sense cells are to be grouped together vdth 

 the ganglion cells of general visceral and gustatory fibers as pri- 

 mary visceral receptive cells, while the rod and cone cells of the 

 retina and the ganglion cells of the general cutaneous and acustico- 

 lateral fibers are primary somatic receptive cells. The cells of the 

 two categories are broadly homologous, as primary receptive cells 

 possessing neurites. The taste cells and neuromast cells are ac- 

 cessory sense cells without neurities. Turning to the central 

 nervous system, a certain connection between the gustatory and 

 olfactor}^ apparatus is at once evident since the chief tertiary tracts 

 of both systems enter the hypothalamus. When the structural 

 relations of the correlating centers of the brain are examined in 

 the following chapters, evidence vdll appear that all of the gusta- 

 tory and olfactory centers belong to a continuous zone of the 

 brain of which the visceral lobe in the medulla oblongata is 

 a part. It is necessary that organs which function as visceral 

 sensory organs should be related to the same set of central nuclei. 



DEMONSTRATION OR LABORATORY WORK. 



1. Study the development and histology of the olfactory organ in 

 fish, amphibian or chick embryos by the method of Golgi. 



2. Study the histology of the olfactory bulb in the brain of a fish 

 or frog. Golgi method. 



3. Study the arrangement of the secondary nuclei and tracts in 

 dissections of the brains of a selachian and a frog and in serial sections 

 by the Weigert and Golgi methods. 



