A combination of histology, whole muscle force physiology, glycogen depletion, and venom expulsion analyses using transonic probes to measure venom flow and fluid pressure transducers to measure venom pressure was performed on the m. compressor glandulae and m. pterygoideus glandulae. The m. pterygoideus glandulae has less than one‐third the cross‐sectional area of the m. compressor glandulae, and produces approximately one‐fifth the total twitch and tetanic force; however, in situ surface stimulation of the muscle produces venom flow and pressure levels that are similar to those produced by the m. compressor glandulae. The similarity in venom output following stimulation reflects in part the functional role of the larger m. compressor glandulae in jaw adduction, but also the functional subdivisions within this muscle. The m. compressor glandulae is divided into a series of columnar fascicles that run from the surface of the muscle to the venom gland. The combined results of clearing and staining and glycogen depletion studies suggest that these fascicles may represent functional compartments. Identical stimulations applied to different regions of the m. compressor glandulae result in up to a six‐fold difference in venom expulsion. This functional specialization may play a role in the regulation of venom flow during offensive and defensive strikes. J. Morphol. 246:249–259, 2000. © 2000 Wiley‐Liss, Inc.

Head triangulation is a regular component of the defensive behaviour of both Heterodon and Dasypeltis. In Heterodon platyrhinos, the triangulation is achieved primarily through rotation of the quadrate in the transverse plane with the distal end of the quadrate moving dorsad. The quadrate is pulled by the m. cervicomandibularis acting in conjunction with one of the jaw adductors. Head triangulation in Dasypeltis scabra is produced by rotation of the quadrate mainly in the frontal plane with the distal end of the quadrate moving craniad. Experimental analysis documents that contraction of the m. adductor mandibulae externus super®cialis is temporally linked to this displacement of the quadrate. The presence of two different mechanical bases for head triangulation in these distantly related colubrids suggests that this behavioural display has evolved independently in these ophidian lineages.

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Heterodon platyrhinos normally produces sounds as one part of its elaborate defensive behavior. This species has a distinctive quadriphasic sound, reflecting the underlying triphasic ventilatory pattern of snakes. Heterodon produces sound during both inhalation and exhalation; in both cases the sounds span a broad frequency range (from approximately 1,000 to 11,000 Hz, with frequencies below 3,500 Hz having higher amplitudes. Temporal congruence between sound production and deflection of a strain gauge mounted over the external nares of unrestrained H. platyrhinos provide the first experimental evidence for sound production through the nasal passageway in snakes.