110 ON THE MORPHOLOGICAL CONSTITUTION OF 



animal, they become modified in form in proportion to the 

 withdrawal of the other organic systems, until at last the 

 sclerotome may become a mere nodule or filament. Although 

 it is also generally admitted that a certain amount of deteriora- 

 tion takes place in the sclerotome towards the anterior part of 

 the cranium, the nature and extent of the change has not 

 hitherto been precisely determined. I find that it presents, 

 according as the nasal fossae are, or are not present, two forms. 

 First general form of Deterioration. The deterioration is 

 much less in the first form than in the second. The first form 

 may be best observed in the mammal, in which alone the 

 nasal cavities are complete. The nasal fossae of the mammal 

 are bounded below by a series of at least four haemal arches, 

 the palatine, maxillary, intermaxillary, and ali-nasal, which, 

 along with the soft parts, form collectively the palatal vault 

 of the mouth, with the upper lip and under surface of the ex- 

 ternal nose ; these three continuous surfaces forming in fact 

 the anterior part of the sternal or haemal aspect of the head, 

 the palatal portion being inclosed within the mouth in con- 

 sequence of the elongation of the lower jaw. If now the 

 sclerotome, of which the intermaxillary bones constitute the 

 haemal arch, be examined, it will be found to present supe- 

 riorly the two nasal bones, as its neural elements ; but which, 

 instead of bounding, along with their corresponding centrum, 

 a neural space, assist the intermaxillary bones in forming two 

 spaces, which are completed, and at the same time separated 

 from one another by the centrum, which, no longer separating 

 a neural from a haemal space, separates a pair of lateral neuro- 

 haemal spaces, or nostrils, from one another. This modifica- 

 tion of the sclerotome depends, primarily, on its not being 

 required to enclose a segment of the neural axis ; and, second- 

 arily, on its co-operating in the formation of the nostrils. 

 This form of sclerotome, in which the centrum passes from 

 above downwards, I denominate catacentric, to distinguish it 



