Royal Institution, 



205 



vertebrata generally ; and stated, that after many years' considera- 

 tion given to the subject, he had convinced himself of the accuracy 

 of the idea that the endo- skeleton of all vertebrate animals was ar- 

 ranged in a series of segments, succeeding each other in the direction 

 of the axis of the body. For these segments or "osteocommata** 

 of the endo-skeleton, he thought the term "vertebrae" might well 

 be retained, although used in a somewhat wider sense than it is 

 understood by a human anatomist. The parts of a typical vertebra 

 were then defined, according to the views explained in the Professor's 

 ' Lectures on Vertebrata ' ; and he proceeded to apply its characters 

 to the four segments into which the cranial bones were naturally 

 resolvable. The views of the lecturer were illustrated by diagrams 

 of the disarticulated skulls of a fish, a bird, a marsupial quadruped, 

 and the human foetus. The common type was most closely adhered 

 to in the fish, as belonging to that lowest class of vertebrata in which 

 " vegetative repetition*" most prevailed, and the type was least ob- 

 scured by modifications and combinations of parts for mutual sub- 

 servience to special functions. The bones of the skull were arranged 

 into four segments or vertebrae, answering to the four primary divi- 

 sions of the brain, and to the nerves transmitted to the four organs 

 of special sense seated in the head. Prof. Owen adopted the names 

 which had been assigned to these vertebrae from the bones constitu- 

 ting their neural spines, viz. occipital, parietal, frontal, and nasal; 

 and enumerated them from behind forwards, because, like the verte- 

 brae of the tail, they lose their typical character as they recede from 

 the common centre or trunk. The general results of the Professor's 

 analysis may be thrown into the following tabular form : — 



Primary Segments of the Skull-hones of the Endo-skeleton, 



VEBTEBa^. 



Centrums. 

 Neurapophyseg. 

 Neural Spinet. 

 Parapophyses. 

 Pleurapophyses. 

 H/tmapophyses. 

 Uamal Spines. 

 Diverging Appendage. 



Basioccipital. 



Exoccipital. 



Supraoccipital. 



Paroccipital. 



Scapula. 



Coracoid. 



Epistemum. 



Fore-limb or fin. 



PARIKTAt. 



Basisphenoid. 



Alispaeuoid. 



Parietal. 



Mastoid. 



Stylohyal. 



Ceratohyal. 



Basihyal. 



Brancniostegals. 



Presphenoid. 



Orbitosphenoid. 



Frontal. 



Postfrontal. 



Tympanic. 



Articular. 



Dentary. 



Operculuni. 



Vomer. 



Prefrontals. 



Nasal. 



None. 



Palatal. 



Maxillary. 



Premaxillary. 



Pterygoids and Zygoma. 



The upper or neural arch of the occipital vertebra protected the 

 epencephalon, or medulla oblongata and cerebellum ; that of the pa- 

 rietal vertebra protected the mesencephalon, or third ventricle, optic 

 lobes, conarium and hypophysis ; that of the frontal vertebra the jaros- 

 encephalon, or cerebral hemispheres ; that of the nasal vertebra the 

 rhinencephalon, or olfactory crura and ganglions. 



The superior development of the cerebral hemispheres in the warm- 

 blooded class, and their enormous expansion in them, occasions cor- 

 responding development of the neural spines, not only of their proper 

 vertebra, but, by their backward folding over the other primary seg- 

 ments, of those of all the other vertebrae ; whilst the more important 



* The general principle of animal organizations, which Prof. Owen has 

 termed "the law of vegetative or irrelative repetition," is explained in the 

 first volume of his * Hunterian Lectures, — on the Invertebrate Animals.' 



