Functional anatomy of the peripheral olfactory system of the African lungfish Protopterus annectens Owen: macroscopic, microscopic, and morphometric aspects.

An anatomical functional study of the olfactory system of Protopterus annectens along the course of the primary olfactory neuron was made by means of macroscopic and microscopic methods on samples of specimens weighing from 3 to 1.250 gm. The olfactory organ, opening by anterior and posterior nostrils into the oral cavity, may be considered a macrosmatic fish type according to the repartition of receptor areas along parallel grooves and on the basis of its active irrigation mechanism. A morphometric study of Protopterus, using a set of seven specimens weighing from 4 to 350 gm, is compared with two morphometric studies on Silurid fishes and reveals some original features concerning the increase of the surface area of the receptor epithelium. The olfactory organ is meant to function in water; its isolation from the aerial environment is carried out by closing the apertures with edema linked to the hypothyroid and cholinergic crisis during the starvation stage; it represents an adaptation to the life in a special environment. This study does not support the assimilation of the posterior intrabuccal opening to a choana, nor the presence of a functional vomeronasal organ. The olfactory nerve shows a definite degree of organization, but it was not possible to recognize any vomeronasal nerve linked with the former. The histological organization of the bulbar relay is of a primitive type; it was impossible to locate an accessory olfactory bulb corresponding to a functional vomeronasal organ, the presence and the innervation of which by the nervus terminalis is discussed.