THE NASAL CAVITIES. 519 



skeleton ; but that of the external wing is very incomplete, in consequence of its 

 being only formed by the inferior extremity of the common cartilage. 



These cartilages, it will be understood, sustain the alte of the nose, prevent 

 their falling inwards, and always keep open the external orifices of the respiratoiy 

 apparatus. 



Muscles. — The motor muscles of the alae are all dilators in the domesticated 

 animals. They are : the trcmversalis dilatator naris, a single muscle placed on the 

 widened portions of the cartilages ; the dilatator naris lateralis, the insertion of 

 which occupies the whole extent of the external wing ; the dilatator naris superior, 

 fixed, by its two portions, to the skin of the false nostril ; the dilatator naris 

 inferior, which is confounded, superiorly, with the external fasciculus of the pre- 

 ceding muscle, it being attached to the inferior branch of the cartilaginous 

 appendix of the supermaxillary turbinated bone ; and, lastly, the levator labii supe- 

 rioris alaque nasi, the anterior branch of which is inserted, in part, into the external 

 wing. All these muscles, having been described in the Myology (p. 278), need 

 not be further alluded to here. 



Skin of the nose. — The skin covering the alae of the nose, externally, is 

 doubled over their free margin to line their internal surface, being prolonged 

 over the entire extent of the false nostril, and is continued in the nasal fossae, 

 by the properly so called pituitary membrane. This skin is fine, thin, charged with 

 colouring pigment, often marked by leprous spots, and adheres closely to the 

 muscles included between its duphcatures, through the medium of a very dense, 

 resisting, connective tissue. 



Vessels and nerves. — The nostrils are supplied with blood by the superior 

 coronary, the external nascd, and the palato-labial arteries ; it is returned by the 

 glosso-facial veins, and partly by the venous network of the nasal mucous 

 membrane. The lymphatics, large and abundant, receive those of the pituitary 

 membrane, and join the submaxillary glands by passing over the cheeks. The 

 nerves are very numerous, the sensory being derived from the maxillary branch 

 of the fifth pair, and the motors from the facial nerve. 



Functions. — The nostrils permit the entrance to the nasal cavities, of the 

 air which is to pass to the lungs. Their dilatability allows the admission of a 

 greater or less volume, according to the demands of respiration. It is to be 

 remarked that, in Solipeds, the nostrils are the only channel by which the air can 

 gain access to the trachea, in consequence of the great development of the soft 

 palate, which is opposed to its entrance by the mouth ; these orifices are therefore, 

 for this reason, relatively larger than in the other domesticated animals, in which 

 the passage of air by the buccal cavity is easily accomplished. 



2. The Nasal Foss^ (Figs. 306, 307, 308). 



Channeled in the substance of the head, above and in front of the palate, and 

 separated from one another, in the median plane, by a cartilaginous septum which 

 does not exist in the skeleton, the nasal fossas extend from the nostrils to the 

 cribriform plate of the ethmoid bone, in a direction parallel to the larger axis of 

 the head. Their length is, therefore, exactly measured by that of the face (see 

 Fig. 308 for the whole of these cavities). 



The nasal fossae are formed by two lateral walls, a roof or arch, di. floor, and 

 two extremities. 



Walls. — The two walls are very close to each other, and the more so as they 

 are examined towards the ethmoid bone and the roof of the cavity. The space 



