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

Abstract

SUMMARY. A general account of the palate, nasal and vomeronasal organs throughout the Squamata is given, since these structures are intimately related to the lachrymal duct. 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 exochoanalis). 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 (soft tissues between the bony palate and the month) 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 groove. are reduced, and in snakes they are entirely absent, so that the superficial palate is complete. The relations of the lachrymal duct to the organ of Jacobson and choanal groove in a relatively large number of squamate types are deseeibed. 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. The ontogeny of the structures considered is described in general terms, and it is shown that 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 vomerinc 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 described 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 mportant 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 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 physiological 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.