THE SKELETON OF THE VERTEBRATE HEAD. 16*7 



timie pervious in all the Mammalia except the Cetacea. The 

 lachrymal canals which connect the anterior pouches of the 

 conjunctive with the nasal fossse, consist of the persistent 

 upper portions of these clefts. Their outer or lower portions 

 are obliterated, but the corresponding inter-scleratomic space, 

 much dilated, constitutes that part 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 portions of the space between these 

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



The posterior nares are not meta-somatomic openings, they t 

 are merely the communications between the catacentric hsemal 

 space of the pre-sphenoidal, and the corresponding but un- 

 divided hsemal 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 transverse 

 plane as the posterior nares. Its fundamental or morpho- 

 logical relations are retained and represented by the posterior 

 isthmus of the fauces. The buccal chamber is a vestibule 

 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 haemal 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 mammalian head, is also a characteristic 

 feature of the nasal fossse in the mammal. As the centrum of 

 that sclerotome, of which the intermaxillaries are the hsema- 

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

 sphenoidal centrum, forming a beam which adds to the antero- 

 posterior strength of the entire arrangement, and which 

 supports the more feebly-developed ethmoidal and rhinal 



