SENSORY ORGANS. . 179 
cially among the terrestrial vertebrates, in which accessory parts are 
present. For details of these reference must be made to histological 
text-books; only a mention of some of the kinds can be made here. 
In the simple tactile corpuscle the nerve terminates with a cup 
in which is seated a lenticular tactile cell (fig. 177, A). Somewhat 
allied are Grandry’s (Merkel’s) corpuscles in which two or more 
tactile cells are enclosed in a connective tissue sheath, while the nerve, 
losing its medullary sheath as it reaches the capsule, expands into 
plates which are inserted between each two tactile cells (fig. 177, B). 
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Fic. 177.—A, tactile corpuscle; Fic. Saeee corpuscle. 
B, Grandry’ S$ corpuscle. 
In another series of sensory structures the end of the nerve is club- 
shaped and is surrounded by a connective-tissue sheath, either simple 
(cylindrical corpuscles), or in Pacini’s (Vater’s, fig. 178) and 
Herbst’s corpuscles, the sheath is formed of layers of cells, recalling 
the coats of an onion, while immediately around the club is a layer 
of cubical cells. Still another variant is found in Krausse’s (corpus- 
culum bulboideum) and Meissner’s corpuscles, where the nerve, 
on entering the corpuscle, breaks up into numerous branches which 
surround an axial core of large cells. 
It is impossible at present to state with certainty the function of 
each of these and other nerve-end apparatuses and to say which are 
connected with the different senses—tactile, pressure, pain, heat and 
cold, muscular, etc.—which are commonly confused under the term 
‘touch.’ 
Lateral Line Organs. 
The lateral line organs occur only in the ichthyopsida and here 
only during the branchiate stages. They arise as thickenings of the 
ectoderm on either side of the head in the neighborhood of the ear. 
From here the thickenings extend in definite lines which determine the 
