THE SKELETON OF THE VERTEBRATE HEAD. 157 



compressed neural space, along the upper part of which the 

 olfactory nerves pass forward. In the posterior superior 

 part of each of these neurapophyseal fibrous laminae, a com- 

 paratively small plate of bone is developed, while the centrum 

 consists of the bar of persistent cartilage, which extends along 

 the grooved upper surface of the anterior portion of the post- 

 sphenoidal centrum, and terminates above the ethmoidal cen- 

 trum (" vomer "). The optic nerves pierce the membranes so 

 far back as to notch very deeply the anterior margins of the 

 post-sphenoidal ueurapophyses, or post-sphenoidal wings. 



In the carp, again, the interorbital space is occupied above 

 by a considerable descent of the margins of the spheiioido- 

 frontal groove ; in front, by complete ossification of the fibrous 

 membranes, which thus become pre-sphenoidal neurapophyses ; 

 behind, by the passage forwards of the post-sphenoidal wings 

 (" ali-sphenoids "), through which, during development, the 

 optic nerves have passed back, to be lodged in notches in their 

 posterior margins ; and below, by the bar of semi-ossified 

 cartilage situated upon the upper surfaces of the posterior 

 sphenoidal and ethmoidal centrums. 



Of the Hcemal Arch and Hcemactinagophyses of the Pre- 

 sphenoidal Sclerotome. The palatine arch, between which and 

 the mandibular the mouth is situated, and which terminates 

 therefore posteriorly the prestomal series of haemal arches, 

 may be presumed to undergo very varied modifications in 

 connection with the olfactory, the respiratory, and the digestive 

 functions. In the present instance, as in many others, the 

 anatomy of the human body, instead of leading astray by 

 complexity and extreme modification of its parts, supplies the 

 key for their morphological solution, by affording an example 

 of the employment of the fundamental type of structure for the 

 fulfilment of the most complex functional purposes. 



The human pre-sphenoidal centrum, hollowed out by nasal 

 air-cells, as in certain birds, is bounded below and in front by 



