1. =The Nasal Cavity. Cavum nasi.=--The osseous framework of the nasal cavity has already been described (page 59), and in connection with this description the boundaries of the cavity and its connections with other cavities have been given. It consists essentially of a large cavity bounded by the facial bones and divided by a longitudinal partition into two lateral halves. The two cavities thus formed are nearly filled by (1) the labyrinths of the ethmoid (ethmoturbinals), (2) the superior nasal conchæ or nasoturbinals, projecting into the dorsal part from the ventral surface of the nasal bones, and (3) the inferior nasal conchæ, or maxilloturbinals, projecting into the ventral portion from the medial surfaces of the maxillaries. There remain to be considered, in addition to the bones, the =cartilaginous= framework of certain parts of the nose, and the =mucous membrane=. The lamina perpendicularis is continued by cartilage, especially craniad, in such a way as to make a complete septum separating the two cavities. This septum extends from the septum of the external nose caudad to the lamina cribrosa, and from the internasal suture ventrad to the vomer and the suture of the premaxillaries. All parts of the nasal cavity are lined by mucous membrane. This is continuous at the nares with the integument, while at the choanæ it passes into the mucosa of the pharynx. It covers the conchæ nasales and the labyrinths of the ethmoid, passing into the cellules of the latter. Owing to the crowding together of the conchæ nasales and the labyrinths the nose is almost completely filled, only three narrow passageways being distinguishable. The ventral one of these, known as the =ventral= or =inferior meatus= of the nose, lies ventrad of the inferior nasal concha, next to the nasal septum. It passes caudad beneath the horizontal plate formed by the vomer and ethmoid, and opens caudad at the =choanæ= into the nasopharynx. The =dorsal= or =superior meatus= of the nose lies just ventrad of the superior nasal concha, next to the median septum; it leads to the frontal sinus, the lamina cribrosa, and the caudal parts of the ethmoid. The =middle meatus= of the nose has almost disappeared in the cat; it is simply the narrow space between the superior and inferior meati. It is practically filled by the ethmoid, into the cells of which it leads. The mucosa is continued from the nasal cavity into the sinuses of the frontals and presphenoid. At the sides of the nasal septum, near the ventral edge, and about one to one and a half centimeters caudad of the nares, there is on each side a small curved cartilaginous tube, about one centimeter or less in length. This, the =vomeronasal= organ, or =organ= of =Jacobson=, lies against the septum, between it and the mucosa. It begins at the incisive canal or anterior palatine foramen, in the roof of the mouth, curves thence caudodorsad close against the side of the nasal septum, and ends blindly in the nasal cavity.