ON THE VERTEBRATE SKELETON. 



289 



by its restricted functions, and having no call for progressive growth in sub- 

 serviency to a branchial respiration. It consists of a ligamentous stylohyal, 

 its pleurapophysis, retaining the same primitive histological condition which 

 obstructs the ordinary recognition of the same elements of the lumbar haemal 

 arches. The hsemopophyses and haemal spine are, however, here as there, 

 more advanced in respect of their tissue. The hcemapophysis is ossified like 

 the so-called ' abdominal ribs,' and usually, like them, consists of two portions, 

 having the special names of epihyal (39) and ceratohyal (40) : the haemal 

 spine (41) retains its cartilaginous state like its homotypes in the abdomen : 

 there they get the special name of' linea alba' or abdominal sternum, here 

 of ' basihyal.' With respect to formal modification, this element is chiefly 

 remarkable in the crocodile for its broad expanse : it sustains the ascending 

 valvular ridge at the base of the tongue, which, applying itself against the 

 descending ' palatum molle,' constitutes an effectual barrier against the entry 

 of water into the glottis from the mouth, whilst the crocodile is engaged in 

 overcoming the struggles of a submerged and drowning prey. 



There being no need of diverging appendages from the hyoidean arch in 

 the crocodile, brauchiostegal rays are not developed. The scapular arch is 

 similarly simplified in Anguis and other serpentiform lizards ; but, to those 

 who recognise its true homology, its presence without a trace of its appen- 

 dages, the fore-limbs, will create no more surprise, than the presence of the 

 hyoidean arch without the branchiostegal fins or of the mandibular arch without 

 the opercular fins. 



On removing the neural arch of the parietal vertebra, with or without the 

 section of the connate centrum (n), the bones completing, with the part (9), 

 the corresponding arch of the frontal vertebra present the general arrange- 

 ment shown in fig. 20. 



The compressed produced 

 bone, 9, shown in natural con- 

 nection with the bone 10 in 

 fig. 9, notwithstanding its mo- 

 dified form, presents all the 

 essential characters of the cen- 

 trum of the arch : although it 

 may have been developed ex- 

 clusively from the capsule of 

 the notochord, like the coa- 

 lesced inferior parts of the cer- 

 vical centrums in the silurus : 

 there is no distinct ossicle an- 

 swering to the central part of 

 the centrum of the frontal ver- 

 tebra, like 9', fig. 5, in certain 

 bony fishes. On theotherhand, 

 wefindthe neurapophysial cha- 

 racters of the orbito-sphenoids 

 (10, 10) more largely and typi- 

 cally manifested in the croco- 

 dile : they are smoothly excavated within by the sides of the prosencephalon : 

 they dismiss the great special-sense nerves of the eye by the notch (fig. 9, op), 

 and the motor nerves by the notch s : they show, however, the same ten- 

 dency to change of position as the succeeding neurapophyses ; for though 

 they support a greater proportion of their proper spine (u), they also sup- 

 port part of the succeeding spine (7), and rest below in part upon the pa- 



Fig. 20. 



Disarticulated prosencephalic arch, viewed from 

 behind: Crocodile. 



