68 



MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



mandibularis externus branches of the seventh nerve, those on 

 the trunk by the lateralis branch of the tenth nerve. These 

 organs occur in the aquatic stages of the amphibia ; but upon 

 the assumption of terrestrial life, as in salamanders, frogs, etc., 

 the organs are lost and their nerves disappear. 



In selachians and ganoids are found, especially on the snout, 

 other sense organs, known as ampullae and Savi's vesicles, the 

 functions of which are more problematical than even those of 

 the lateral line organs. The ampullae may be organs of pres- 

 sure sense. 



Allied to the sense organs of the lateral line are structures 

 known as end buds. These consist of a number of sensory 



cells, each bearing sen- 

 sory hairs, compacted 

 into a bud-like mass, and 

 surrounded by supporting 

 cells. In the cyclostomes 

 and fishes they are scat- 

 tered over the surface, 

 but from dipnoi upwards 

 they are confined to the 

 cavities of the mouth 

 and nose, and in the 

 higher vertebrates to the 

 oral cavity. In the mam- 

 mals, these function as organs of taste, and the same is prob- 

 ably true of the lower vertebrates, since certain fishes have been 

 shown to be capable of tasting with the external skin. 



Sense Corpuscles. In the terrestrial vertebrates the epi- 

 dermal sense organs take a great variety of shapes due to the 

 modifications of the accessory structures sometimes unicellu- 

 lar, sometimes multicellular in character ; but in all these we prob- 

 ably have to do with free nerve terminations on or between 

 the accessory cells. In all cases these structures are buried in 

 the deeper layer of the epidermis or in the dermis beneath. 

 The simplest are oval cells, the deeper face of each seated in 

 a cup-like expansion of a nerve termination. In the compound 

 tactile cells (Grandry's or Merkel's corpuscles), found only in 



FIG. 69. Taste buds (end buds) from the 

 human mouth (from Martin). 



