Page:Popular Science Monthly Volume 4.djvu/607

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in our considerations every class of animals in which the specialization to be considered appears. For example, every animal belonging to the vertebrate sub-kingdom of animals agrees with every other animal of the same sub-kingdom in the following distinctive characteristics—characteristics, too, that we shall find involved in our considerations of the subject of the facial angle: In all, the head and vertebral column are composed of a number of definite segments, arranged along a longitudinal

Fig. 1.
PSM V04 D607 Cranial profiles.jpg

axis; each segment of this framework is normally composed of a body and two diverging, ring-like formations; the dorsal containing the brain and spinal cord, the ventral, or abdominal, containing the organs of nutrition, as the alimentary canal, circulating and eliminating organs. Every vertebrate animal, then, is possessed of two tubes of framework: the one, to protect the brain and spinal cord; the other, the organs of nutrition. These tubes are subject to very great variation, and are modified, as by a master's hand, to meet the necessities that their various specializations of function may demand. The great modification in the calibre of the dorsal tube in different classes of the vertebrates, as well as the great variation in shape of the elements which compose that arch, is apparent to every one. In the region of the spine, the elements that compose the segments of the arch are rounded, and at some distance apart, while in the cranial (skull) region they are flattened, spread out, so as to unite and form sutures, thus making a solid brain-case, for the protection of the softer and more massive nerve-matter.