206 Royal Institution, 



parts of the neural arch, as the neurapophyses, undergo comparatively 

 little change. 



The acoustic nerve escapes between the occipital and parietal ver- 

 tebrae, but the organ itself is intercalated between the neural arches 

 of these segments and its ossified capsule ; the petrosal projects into 

 the cranial cavity between the exoccipital and alisphenoid in the 

 warm-blooded vertebrata. The gustatory nerve (part of the third 

 division of the fifth pair) perforates or notches the alisphenoid, and 

 in crocodiles and many fishes passes through an intervertebral fora- 

 men between the alisphenoid and orbitosphenoid ; but the gustatory 

 organ is far removed from the neural arches or cranium proper, and 

 is united with its fellovv^ to form the apparently single organ called 

 the tongue. The optic nerve perforates or grooves the orbitosphe- 

 noid, and the eyeball intervenes between the frontal and nasal ver- 

 tebrae, as the earball does between the occipital and parietal : the 

 vertebral elements are modified to form cavities for these organs of 

 sense ; that lodging the eye being called the " orbit," that for the 

 ear the " otocrane." 



The divergence of the olfactory crura, and the absence of any union 

 or commissure between the olfactory ganglia, leads to an extension 

 of ossification from their neurapophyses, which are always perforated 

 by the olfactory crura or nerves, to the median line between those 

 parts ; and the neurapophyses themselves coalesce together there in 

 batrachia, birds and mammals. This extreme modification was to 

 be expected in a vertebra forming the anterior extremity of the se- 

 ries ; and the typical condition of the prefrontals, so well shown in 

 fishes and saurians, is marked in mammals by the enormous deve- 

 lopment of the capsules of the organ of smell anterior to them, which 

 become ossified and partially anchylosed to the compressed, shrunken 

 and coalesced prefrontals; the whole forming the composite bone 

 called "sethmoid" in anthropotomy. The vomer, or body of the 

 nasal vertebra, has undergone an analogous modification to that 

 which the terminal vertebra of the tail presents in birds ; whence its 

 special name, referring to the likeness to a ploughshare, in human 

 anatomy. The spine, or nasal bone, is sometimes single, sometimes 

 divided, like the frontal, the parietal and the supraoccipital bones. 

 Their special adaptive modifications have obtained for them special 

 names. 



The haemal arches corresponding with the above neural arches 

 retain most of their natural position and proportions, as might be 

 expected, in fishes ; they are called the scapular, hyoid, mandibular 

 and maxillary arches. The pleurapophysis of the occipital vertebra 

 is the scapula, and is commonly attached by a head and tubercle to 

 the centrum and parapophysis of its proper occipital vertebra. 



The hyoid arch is suspended by the medium of the epitympanic to 

 the mastoid parapophysis of the parietal vertebra, the epitympanic, 

 in fishes, intervening and separating the haemal arch from its proper 

 vertebra, just as the squamosal intervenes to detach the tympanic 

 pleurapophysis of the mandibular arch from its proper vertebra in 

 mammals ; which vertebra the squamosal attains in man by articu* 



