548 COMPARATIVE ANATOMY. 



§412. 



There are accessory organs belonging to the nasal cavity. These are : 

 1) Accessory cavities of the nose. These are formed by 

 the sinking of the mucous membrane of the nose into parts of its 

 firm wall. They are first seen in the Crocodilini, where there is a 

 cavity in the side walls of the nasal cavity, which communicates 

 with it. In Birds we frequently meet with connections between the 

 nasal cavity and the spaces in the neighbouring bones. In the Mam- 

 malia the nasal cavity communicates with a number of cavities in 

 different bones of the skull, the most important of which are the sinus 

 frontales. These are cavities which are placed in the frontal bone, 

 and which are either single, or divided into smaller portions ; they 

 are very greatly developed in the Ruminantia. There are other 

 communications with the sphenoid ; these are greatly developed in 

 the Elephant, for example, where the cavities extend through the 

 parietal and temporal bones as far as the occipital condyles. Lastly, 

 there are connections between the nasal cavity and the maxilla ; 

 these form the sinus maxillaris, which is developed in Marsupials 

 and Ruminants, and very largely in the Solidungula. In Pi-iniates 

 they are less extensive, and they are not present in most Carnivora, 

 Edentata, or Rodentia. 



2) Glands. There are larger glands connected with the nasal 

 cavity -in addition to the glandular structures which are ordinarily 

 found on the mucous membrane of the nose. When they are more 

 developed they may also extend outside the nasal cavity. Such 

 nasal glands are found in the Amphibia and in the Ophidii, as also 

 in the Saurii and Crocodilini; in the former they lie outside the 

 upper jaw, and in the latter they are enclosed in a maxillary sinus. 

 In Birds also there is an external nasal gland, which is sometimes 

 placed on the frontals, and sometimes on the nasal bones. Among 

 the Mammalia also we find a gland on the sides of the face, but it is 

 absent in several orders. 



3) Organ of Jacobson. This is a canal placed at the base of 

 the nasal cavity ; it is generally attached to the nasal septum, and 

 communicates at the palate with the buccal, though it is shut off 

 from the nasal, cavity; its walls, which form various kinds of pro- 

 cesses, carry the ends of a branch of the olfactory nerve, which 

 passes down the sides of the septum. In the Ophidii and Saurii the 

 canal is partly enclosed by the vomer; in the Mammalia these 

 organs are elongated, and are continued, as the ducts of Stenson, 

 through the incisor canals, to the surface of the palate ; they are 

 best developed in the Ruminantia and Rodentia (§ 396). 



Buccal Cavity. 



§ 413. 



When the primitive antechamber of the enteric tube is divided 

 into the nasal and buccal cavities, by the formation of a palate, a 



