| OCR Text |
Show 294 ON 'l'HE STRUC'l'UH~ OF 'l'HE VERTEI3RA~rE SKULl,, body of a vertebra-such as normally each caudal :erteb:·a of a M·:t1n1nal is · and that frmn this, for the purpose of 1nvestmg tho olfactory ap~aratuses, which are developed at ~ts sid:s, lamellar processes grow out, which are altogether pecuhar_to It. ~n any case, however, the ethn1oid may be regarded as the antenor end of the vertebral column. "(10.) JTrmn what has been stated, it appears that the four different groups of boues-the occipital, with its intercalary bone, the squama; the basi-sphenoid, with its intercalary bone, tho parietals; the presphenoid, with its intercalary bones, the frontals ; and the ethmoid, together with its outgrowths, tho spongy bones and the cribriform plate-exhibit in their successive order from behind forwards, a greater and greater deviation from the plan according to which ordinary vertebroo are developed, so that the occipital bone is most like a vertebra, while the eth1noid is least like one. H (11.) Among the bones of the face, the premaxillce, the nasal bones, and the vomer are developed altogether independently of the investing mass of the notochord; and they never coalesce with parts of the skeleton, which are immediately derived from the latter. On this account, alone, they cannot be regarded as vertebrre, or parts of vertebrre. Furthermore, they at no time enclose, or help to enclose, a segment of the central nervous system. The nasal bones and the vomer are, properly speaking,' splint-bones' (Belegungsknochen) for the ethmoid, such as occur in the vertebrre of no animal ; and the premaxillro are applied, although in a different plaue, to the one end of the vertebral column, as, in Fishes, the median rays of the anal fin are applied to the other end of it.* Furthermore, the palatine bones are developed, together with the pterygoids, in lateral processes, or rays, which have grown out from the middle part of the base of the brain-capsule, and which, as regards their original form, disposition, and connections, resemble the ribs, and may be regarded as a pair of ribs united with the brain-case. In Mamrnalia the two mallei are developed in these two rays, and * The study of the development of the skull necessitates the assumption that Sturgeons, Sharks, aml Rays have no premaxillre, and that their skulls end anteriorly with the ethmoid cartilage. THE 'l'HEORY OF THE VEH'l'EBRA'l'E SKULL. 295 perhaps the quadrate bones of many other Vertebrata in a part of .thmn. Around them, however, is developed, in animals provided with an osseous skeleton, a coating of bony plates, which becomes metmnorphosed into the lower jaw. "At the outer side of those parts, n1oreover, in which the pterygoid and palatine bones arise-or, in other words, alongside the processes of the 'rays '-a substance arises, whence the upper maxilla and the 1nalar bone are developed. "The upper maxilla and malar bone therefore might be regarded, like th0 lower maxilla, as splint bones or rib-like bones (which, however, do not occur in connection with true ribs), but not as parts of the vertebra itself.* The lachrymal bone, lastly, only fills up a gap between other bones of the face, and therefore, if analogies 1nust be discovered, can only be 1·egarded as an intercalary bone. " (12.) The auditory capsules anu the petrosal bones, which are developed out of them in n1any animals, may, in respect of their place and origin, be most fittingly compared with thos.e interealary bones which occur in Sharks and Sturgeons, between the arches of the vertebrm ; but, in respect of their form, take a different course from these. And since those intercalary pieces can hardly be considered to be parts of vertebrm, the auditory capsules cannot be regarded as such." Vogt and Agassiz, resting upon en1bryological observations which entirely confirmed those of Rathke, carry out the argument suggested by the latter more rigorously. "It has therefore become my distinct persuasion (says Vogt) that the occipital vertebra is indeed a true vertebra, but that everything which lies before it is not fashioned upon tho vertebrate type a.t all, and that all efforts to interpret it in such a way are vain; that therefore, if we except that vertebra ( occipital) which ends the spinal column anteriorly, there are no cranial vertebrre at all." t . * In the Chelonia and a few Mammalia bony elements occur, which cover the ribs and, in the first-mentioned animals, even become united with the rib.:>; they are dovolopctl, however, in the integument, and belong to tlte integumenta.ry skeleton, and not to the nervous skeleton, so tllat they ncotl not be consic.lored hero. t "Eutwickolungs-Geschichte dcr Gc.:Lnrt.:>hclii.r Kroto."-P. 100. 1S·l2. |
