THE SKELETON OF TlIK VERTEIiKATE HEAD. ICT 



tinue pemous in all the Mammalia except the Cetacea. The 

 lachrymal canals which connect the anterior pouches of the 

 conjnnctivie with the nasal fossie, consist of the pei-sistent 

 upper portions of these clefts. Tlieir outer or lower portions 

 are obliterated, but the corresjionding inter-scleratomie space, 

 much dilated, constitutes that pait of the orbit formed by 

 elements of the ethmoidal and pre-sphenoidal sclerotomes, 

 while the spheno-palatine and posterior palatine foramina and 

 fissures are also enclosed poilions of the space between these 

 two sclerotomes, retained for the passage of vessels and neiTes. 



The posterior nares are not meta-soraatomic openings, they 

 are merely the communications between the catacentric Inemnl 

 space of the pre-sphenoidal, and the con-esponding but un- 

 divided luemal spaces of the succeeding somatomes. 



The mouth is the persistent and developed form of the 

 great cleft between the pre- and post-sphenoidal somatomes. 

 It is situated therefore morphologically in the same transveree 

 plane as the posterior nares. Its fundamental or moqjho- 

 logical relations are retained and represented by the posterior 

 isthmus of the fauces. The buccal chamber is a vestibide 

 superadded to the alimentary tube, by the anterior elongation 

 of the lower jaw, and by the development of the floor of the 

 mouth and of the tongue, with the consequent inclusion of 

 the vault of the palate ; so that the latter, instead of forming 

 the anterior portion of the hiemal or sternal aspect of the head, 

 becomes apparently a portion of the wall of the visceral tube. 



The complete development of the vomer, characteristic, as 

 already stated, of the mannnalian head, is also a characteristic 

 feature of the nasal fossie in the manmial. As the centrum of 

 that sclerotome, of which the intenuaxillaries are the luema- 

 pophyses, it extends back from them to abut against the pre- 

 sphenoidal centrum, forming a Ix-ani wliich adds to the antero- 

 posterior strength of the entire arnuigement, and wliich 

 supports the more fei'bly-deveIoi)ed ethmoidal and rhinul 



