1. The histological composition of the sensory membrane in the facial pit of rattle-snakes (Crotalus spp.), the disposition of the nerve-trunks entering it, the fibre-size spectrum of the nerves, and the form of the sensory endings are described.
2. Between the two layers of extremely attenuated epidermis the principal constituent of the membrane is a single layer of specialized parenchyma cells with osmiophil, reticular cytoplasm. These are not regarded as sense cells but they react strongly and locally to degeneration of nerve-endings.
3. The axons enter through numerous trunks from three branches of the trigeminal nerve, from all sides of the membrane, providing a dense innervation. They lose their myelin, taper to about 1µ, then expand into flattened palmate structures which bear many branched processes terminating freely over an average area of about 1,500µ2, overlapping only slightly with adjacent units but leaving virtually no area unsupplied. This means there are from 500 to 1,500 axons ending per mm2, an estimate which agrees with the nerve-counts. No other form of ending was found.
4. The mode of the fibre-size spectrum lies in the region 5-7µ, diameter.
5. A transmission spectrum of the fresh membrane shows broad absorption peaks at 3 and 6µ and about 50% transmitted in other regions out to 16µ. The visible spectrum is at least 50% transmitted and probably much is lost by reflection. Strong absorption takes place at wavelengths shorter than 490µ
6. The anatomical adaptations of the sense organ are discussed, especially the concentration of warm receptor fibres, the thinness of the membrane, the extremely superficial position of the nerve-endings--all increasing sensitivity to caloric flux. The overhanging margins of the pit and the richness of supply are believed to permit directionality of reception.
7. It is suggested that the palmate form of the ending has a significance in permitting several independent local sub-threshold activity generators to coexist in the processes and in pooling their coincident, electrotonically spread potentials to influence the initiation of spikes which may take place at the junction of axon and palm.
Modelling mechanotransduction in primary sensory endings
