SENSORY ORGANS. 



I 79 



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). 



FIG. 177.-!, tactile corpuscle; 

 B, Grandry's corpuscle. 



FIG. 178. Yater-Pacinian 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 



