VARIATIONS IN SHAFT STRUCTURE — CILIA 35 



Structure which is more or less rigidly connected to the tympanic 

 membrane. The cilium contains a ring of nine fibril doublets, 

 but the centre of the axial bundle is empty except for an irregular 

 fibrous structure in a dilated region towards the distal end. From 

 the basal body of the cilium nine striated roots run downwards for 

 a short distance (PL VId) before joining to form a thick single root 

 which runs deep (80 to 100 /x) into the dendrite with repeated 

 branching. Here again the cilium has a conducting function, and 

 indeed seems to generate the receptor impulse. 



Other cilia which could have a sensory function are those on the 

 crown cells in the saccus vasculosus of fish. Here again the central 

 fibrils are missing and the cilia are non-motile. The free part of 

 the cilium is club-shaped and the peripheral fibrils which are 

 obvious near the base extend towards the tip as strings of vesicles 

 (Bargmann and Knoop, 1955; Porter, 1957). The function of 

 these cilia is not obvious, but, since the saccus vasculosus is 

 believed to act as a pressure receptor, the bulbous cilia may be the 

 receptor organs. 



The cnidocils of nematocyst cells of Hydra have been shown by 

 Chapman and Tilney (1959) to have a central dense core with 

 nine peripheral ridges which may represent the peripheral fibrils 

 of a modified cilium. At the base of this structure is a typical 

 basal body. 



Wersall (1956) found normal cilia as well as '' stereocilia " 

 (see p. 74) on the hair cells of the cristae ampullares in the guinea 

 pig ear. The normal cilia contain the full complement of nine- 

 plus-two fibrils and run for most of their length embedded in the 

 gelatinous material of the cupola. It is thought that movements 

 of the cupola, caused by movements of fluid in the ampullar 

 cavity, distort the cilia and cause electrical discharge in the sensory 

 nerve. Cilia with the full number of fibrils are also found on the 

 olfactory rods — the receptive endings of the olfactory nerves — of 

 the rabbit (de Lorenzo, 1957). 



An important skeletal function is performed by certain 

 supporting cilia in protozoa. Ciliary rootlet structures are 

 sometimes enlarged to form skeletal structures in protozoan cells, 

 e.g. the kinetodesmata of Metaradiophrya (p. 63), the costa of 

 Tritrichomonas (p. 71), and occasionally the cilia themselves may 

 form a main part of a skeletal structure. The stalks of several 



