. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. OLFACTORY ORGAN 197 The primary origin of the olfactory organs is by no means understood : possibly it may have arisen by a modification of primitive integumentary sense-organs. It is doubtful whether the organ can be said to have a true olfactory function in Fishes and perennibranchiate Amphibians. In its simplest form, the olfactory organ consists of a ventral, paired, pifc-like depression of tlie integument of the snout opening on to the surface by an external nostril. It is lined by an epithelium which is connected
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. OLFACTORY ORGAN 197 The primary origin of the olfactory organs is by no means understood : possibly it may have arisen by a modification of primitive integumentary sense-organs. It is doubtful whether the organ can be said to have a true olfactory function in Fishes and perennibranchiate Amphibians. In its simplest form, the olfactory organ consists of a ventral, paired, pifc-like depression of tlie integument of the snout opening on to the surface by an external nostril. It is lined by an epithelium which is connected with the brain by the olfactory nerves. The olfactory mucous membrane contains sensory cells, or olfactory cells proper—usually provided with sensory hairs, separated by isolating or supporting cells, both kinds having a smilar origin (Fig. 156). These olfactory cells are said to constitute the only true neiifo-epithelinm in Vertebrates, as the nerve arises in connection with the cell itself, witli whicli it remains continuous, as in many Invertebrates {primanj sensory cell, Retzius). Tlie cell is therefore not merely surrounded by a nerve-network as in other secondfit-;/nerve-cells, and the olfactory organ thus probably represents a very ancient structure phylogenetically. It is possible, however, tliat the central cells of the in- tegumentary sense-organs of Anamnia ( end- buds) may be directly continuous with their nerves, although surrounded by a nerve-network. From the Amphibia onwards glandular elements are present, the secretion of which serves to keep the nasal cavity moist. The olfactory organs of all the true Fishes exhibit the above-described simple sac-like form, but from the Dipnoi onwards they come to communicate with the cavity of the mouth as well as with the exterior. In consequence of this, anterior or external, and posterior or internal nostrils (choance) can be distinguished,. and as a free passage is thus formed through which air can pass, the olfact
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
