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The Anatomy of the Infra-red Sense Organ in the Facial Pit of Pit Vipers

1957; The Company of Biologists; Volume: S3-98; Issue: 42 Linguagem: Inglês

10.1242/jcs.s3-98.42.219

1477-9137

Theodore H. Bullock, Wade Fox,

Amphibian and Reptile Biology

ABSTRACT The histological composition, of the sensory membrane in the facial pit of rattlesnakes (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. 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. 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 1 p, then expand into flattened palmate structures which bear many branched processes terminating freely over an average area of about 1,500 p.2, overlapping only slightly with adjacent units but leaving virtually no area unsupplied. This means there are from 5 00 to 1,5 00 axons ending per mm2, an estimate which agrees with the nerve-counts. No other form of ending was found. The mode of the fibre-size spectrum lies in the region 5-7p diameter. A transmission spectrum of the fresh membrane shows broad absorption peaks at 3 and 6 p and about 50% transmitted in other regions out to 16p. The visible spectrum is at least 50% transmitted and probably much is lost by reflection. Strong absorption takes place at wavelengths shorter than 490 p. 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. It is suggested that the palmate form of the ending has a significance in permitting several independent local subthreshold 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.