Dr. Humphry on the Vertebral Theory of the Skull. 375 



the members of these classes exhibit. This constitutes the branch 

 of anatomy called " Homology." The general features of the plan 

 upon which vertebrate animals are constructed are clear enough in 

 all of them. Osseous segments, or vertebra?, with neural and vis- 

 ceral processes, enclosing respectively the neural and visceral cen- 

 tres, constitute the trunk, including neck, chest, loins, &c. Proba- 

 bility is in favour of the view propounded by Goethe and Oken, and 

 worked out by Oken and Owen, that the skull falls in with the Law 

 of Uniformity, and corresponds with the rest of the frame in having 

 a vertebral composition. It is by all anatomists admitted to be 

 segmentally constructed. Most anatomists are agreed as to the 

 number of segments. Ought not, therefore, these segments to be 

 described by the same name as those of which they form a conti- 

 nuation, especially as they bear the same relations to the neural 

 and visceral centres, and the same or nearly similar relations to the 

 nerves and blood-vessel ? In their mode of development, too, the 

 segments of the skull show a marked general correspondence with 

 those of the trunk. The chorda dorsalis, around which the verte- 

 bral centres are formed, extends at any rate halfway along the base 

 of the skull ; and the bodies and arches of the cranial segments are 

 evolved from a continuation of the same embryonic structure (the 

 " vertebral plates ") as the trunk segments — the chief difference 

 being that in the trunk segmentation takes place at an earlier period 

 than in the head. In the trunk, it is observed in the vertebral plates ; 

 and these primitive segments are called " protovertebrse." They ap- 

 pear not to exist in the head. The segmentation, however, takes 

 place in the cranium as soon as ossification begins, even if it does 

 not do so before ; and the significance of the protovertebrse as dis- 

 tinctive features between the skull and the trunk is diminished, 

 first, by their being related to the formation of the nerves as much 

 or more than to that of the vertebra? ; and secondly, by their not 

 really corresponding with the vertebra?, each permanent vertebra 

 being formed by a half of two protovertebrse. Dr. Humphry expa- 

 tiated on this and other points in the development of the skull, and 

 expressed his decided opinion that the differences between the deve- 

 lopment of it and of the trunk vertebrae were by no means sufficient 

 to controvert the view — which coincides with the Law of Uniformity, 

 and which is confirmed by the segmental construction of the skull, 

 by the relation of its components to surrounding parts, and by so 

 many fundamental resemblances in development — that the same 

 name may be applied to the segments of the skull and of the trunk, 

 and that the one, as well as the other, consists of vertebra? modified 

 to meet the requirements of the parts in which they are found. He 

 concluded by stating that the greater number of those anatomists to 

 whose observations we are indebted for most of our knowledge of 

 the development of the skull and of the trunk, are agreed that the 

 differences between the mode of formation of the segments in the 

 two form no real argument against the vertebral character of either ; 

 and he thought stronger reasons must be adduced than had yet been 

 shown before the anatomists could be called upon to abandon the 

 vertebral theory of the skull. 



