ON THE THEORY OF THE VERTEBRATE SKULL 6oS 



our nomenclature of the parts of the dorsal and lumbar vertebrse throughout the 

 vertebrate series, requires a thorough revision. 



To this subject I hope to return on a future occasion. 



VII. — I subjoin the views of Vogt, and the criticisms of the late great anatomist 

 Johannes Muller upon them, as the best means of e.xhibiting their relation to those 

 I have advocated. 



" If we ask ourselves what we mean by vertebras, the primary segments of the 

 still indifferent tissue round the chorda, which arise in all vertebrate embryos, are 

 the first thmgs to suggest themselves. These persist only in the lowest grades of 

 vertebrate animals, while in the higher they disappear, in consequence of the more 

 and more complete development of secondary organs, especially of the ex- 

 tremities ; so far as we are able to trace these segments, so far is there a formation 

 of vertebrse. 



" But there is at once a difficulty, when we endeavour to find these segments 

 in the rudiment of the skull of any vertebrate embryo. It is true that many 

 inflexions may be observed which appear to correspond with such vertebra, but 

 unfortunately these do not appear in the same places in different embryos ; and 

 besides, these inflexions and curvatures of the base of the skull are not in the least 

 similar to the sharply and clearly defined intervals between the primary vertebrae.. 

 The first of these intervals is always formed behind the auditory vesicles, and lies 

 therefore between the occiput and the first cervical vertebra ; further forwards, 

 as has been said, no such interval is discoverable. But in the Cyclostome fishes, 

 which represent this embryonic condition, no vertebral divisions of the skull 

 are discernible ; in fact we have in the Myxinoids, only the chorda with its 

 sheath and muscular and cutaneous vertebral rings, which are repeated up to 

 the skull, but there cease. The skull of the Myxinoids, like that of the higher 

 cartilaginous fishes, cannot by any amount of violence be forced under the verte- 

 brate type. In the skull, then, the primary vertebral segments are wanting. 

 However, they might be obliterated by the early development of the organs of 

 sense, or by the aberrant development of the brain. 



" But there remains a second means of discovering the cranial vertebra, by 

 examining the solid cartilaginous and bony basis of the skull ; though here also 

 we meet with insuperable difficulties. As the primitive type of the more solid 

 bodies of the vertebra:, we have everywhere cartilaginous rings arising out of 

 the sheath of the chorda, and deposited around its nucleus. Whether they arise 

 as lateral halves or as entire rings, whether they embrace the chorda completely 

 or only above or below, is a matter of no essential moment. But are such carti- 

 laginous rings deposited around the chorda, discoverable in the skull ? They 

 will be sought for in vain unless it be in the last, occipital, cranial vertebra ; in 

 this we still find all the characters of a vertebra — the investment of the chorda, 

 the chondrification in the sheath of the chorda. But the chorda does not pass 

 into the so-called first and second cranial vertebrae ; it invariably ends, as Rathke 

 justly states, between the auditory capsules, and never passes into the body of 

 the second cranial vertebra, let alone that of the first. The lateral cranial 

 trabecule, which bear the two anterior cranial vertebrae, can by no possibility 

 be regarded as centra of vertebrae, since in this case the characteristic feature, 

 the being traversed by the chorda, is entirely absent. Again, these lateral 

 trabecute are continued uninterruptedly forwards, below the first division of the 

 brain, showing no trace of a median division. But in what part of the vertebral 

 column has it ever been seen that two vertebrse arise united and afterwards 



divide ? 



" It has therefore become my distinct persuasion that the occipital vertebra is. 



