GENERAL LAWS OF ANIMAL STRUCTURE. 



26j 



the series, sometimes by enlargement, sometimes by 

 diminution and even disappearance, sometimes by con- 

 solidation of several together. But throughout these 



.../: 



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Fig. 167. — Owen's archetypal vertebrate, showing the successive segments 

 but slightly modified. The olfactory capsule, the eye, and the ear are 

 represented detached. 



modifications the essential identity, although obscured, 

 is still discernible. 



For example: In the dorsal region of the higher 

 vertebrates the visceral arches are greatly enlarged to 

 contain the thoracic viscera. Going forward, in the neck 

 region in man and most vertebrates the visceral arches 

 are wanting in order to give greater freedom of motion, 

 but not in all vertebrates, nor in all stages of develop- 

 ment. Ribs are borne by all the vertebrae up to the 

 head in fishes, in serpents, and also in the embryos of 

 mammals, and even of man. Going still forward, accord- 

 ing to one view, the head is composed of several vertebrae 

 consolidated and greatly modified (see Fig. 167). Ac- 

 cording to this view, the basilar portions of the occipital 

 and sphenoid, and the ethmoid are the centrums of three 

 successive vertebrae, the neural arches of which, greatly 

 enlarged, form the skull, while the diminished and much 

 modified visceral arches make up the face. There is, 

 however, some doubt whether the vertebral theory of 

 the skull is true in this strict sense; and, if so, of how 

 many vertebrae it consists. 



Going now backward, in the lumbar region, again, the 

 ribs are wanting in man and mammals, but not in fishes, 



