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The lachrymal apparatus in lizards and snakes II The anterior part of the lachrymal duct and its relationship with the palate and with the nasal and vomeronasal organs



The lachrymal apparatus in lizards and snakes II The anterior part of the lachrymal duct and its relationship with the palate and with the nasal and vomeronasal organs



Proc Zool Soc London: 269-310



The bony palate of lizards shows variations in the extent to which the adjacent borders of the maxillary palatal processes and vomers are approximated behind the level of the opening of the duct of Jacobson's organ (fenestra vomeronasalis externa), separating this opening from that of the choana (fenestra exo-choanalis). In snakes the bony palate is highly modified, the maxillae are reduced, and in many forms the fenestra vomeronasalis externa is bordered laterally by the septomaxilla. In most lizards the superficial palate is incomplete, the vomerine and maxillary portions being divided by a groove, the choanal groove. This usually terminates anteriorly in relation to the duct of Jacobson's organ, and posteriorly opens into the choana. In some lizards the choanal grooves are reduced, and in snakes they are entirely absent, so that the superficial palate is complete. In all forms the lachrymal duct opens anteriorly in close proximity to the duct of the organ of Jacobson. Behind this level the lachrymal duct may or may not communicate with the choanal groove. Both the duct of Jacobson's organ and the choanal groove represent persistent portions of the primitive choana. The variations in relationships and extent of the choanal grooves are explained mainly on the basis of variations in the degree of fusion between the maxillary and vomerine processes during ontogeny. The condition in Squamata is compared with that in Sphenodon, where the lachrymal duct opens only into the lateral aspect of the nasal sac. It is shown that the essential difference between Sphenodon and the Squamata lies in the forward extension of the lachrymal duct to come into relationship with the duct of Jacobson's organ in lizards and snakes. This involves an invasion by the growing lachrymal duct of the tissues derived from the vomerine process. It is concluded that conditions in Sphenodon are primitive and could readily undergo elaboration along either the squamate or mammalian lines of evolution. Certain functional implications of the conditions descr. are discussed. In the Squamata the organ of Jacobson is more highly developed than in any other vertebrates, and in many lizards the ciliated choanal grooves play an important part in conveying odorous particles to its lumen. In forms where these grooves are small or absent it is usually possible for the particles to be inserted directly by the tongue tips; there is evidence, however, that there may also be some other filling mechanism, the nature of which remains obscure. The constant relationship between the opening of the lachrymal duct and the.duct of Jacobson's organ is thought to have some physiol. significance. The possibility that the secretion of the Harderian gland may possess some special property related to the sensory function of Jacobson's organ is considered.

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