This applies also to the mucous lining of the olfactory organ, the nose. However, the development of this organ is much more interesting. Although the nose seems superficially to be simple and single, it really consists, in man and all other Gnathostomes, of two completely separated halves, the right and left cavities. They are divided by a vertical partition, so that the right nostril leads into the right cavity alone and the left nostril into the left cavity. They open internally (and separately) by the posterior nasal apertures into the pharynx, so that we can get direct into the gullet through the nasal passages without touching the mouth. This is the way the air usually passes in respiration; the mouth being closed, it goes through the nose into the gullet, and through the larynx and bronchial tubes into the lungs. The nasal cavities are separated from the mouth by the horizontal bony palate, to which is attached behind (as a dependent process) the soft palate with the uvula. In the upper and hinder parts of the nasal cavities the olfactory nerve, the first pair of cerebral nerves, expands in the mucous coat which clothes them. The terminal branches of it spread partly over the septum (partition), partly on the side walls of the internal cavities, to which are attached the turbinated bones. These bones are much more developed in many of the higher mammals than in man, but there are three of them in all mammals. The sensation of smell arises by the passage of a current of air containing odorous matter over the mucous lining of the cavities, and stimulating the olfactory cells of the nerve-endings. Man has all the features which distinguish the olfactory organ of the mammals from that of the lower Vertebrates. In all essential points the human nose entirely resembles that of the Catarrhine apes, some of which have quite a human external nose (compare the face of the long-nosed apes). However, the first structure of the olfactory organ in the human embryo gives no indication of the future ample proportions of our catarrhine nose. It has the form in which we find it permanently in the fishes--a couple of simple depressions in the skin at the outer surface of the head. We find these blind olfactory pits in all the fishes; sometimes they lie near the eyes, sometimes more forward at the point of the muzzle, sometimes lower down, near the mouth (Figure 2.249). (FIGURE 2.311. Frontal section of the mouth and throat of a human embryo, neck half-inch long. "Invented" by Wilhelm His. The vertical section (in the frontal plane, from left to right) is so constructed that we see the nasal pits in the upper third of the figure and the eyes at the sides: in the middle third the primitive gullet with the gill-clefts (gill-arches in section); in the lower third the pectoral cavity with the bronchial tubes and the rudimentary lungs.)