. Elements of comparative anatomy. Anatomy, Comparative. BRAIN OF YERTEBRATA. 503 from a mere elongation of a shorter nerve-centre, but from the gradual development of a nerve-chord, which primitively formed a peripheral apparatus only. The differences in the characters of the brain (exclusive of the medulla oblongata) and of the spinal chord, so far as regards the arrangement of the white and gray substance, serve to confirm this view, which is also supported by other facts. A. Central Organs of the Nervous System. a) Brain. § 380. Three successive portions are developed from the rudiments of
. Elements of comparative anatomy. Anatomy, Comparative. BRAIN OF YERTEBRATA. 503 from a mere elongation of a shorter nerve-centre, but from the gradual development of a nerve-chord, which primitively formed a peripheral apparatus only. The differences in the characters of the brain (exclusive of the medulla oblongata) and of the spinal chord, so far as regards the arrangement of the white and gray substance, serve to confirm this view, which is also supported by other facts. A. Central Organs of the Nervous System. a) Brain. § 380. Three successive portions are developed from the rudiments of the brain (Fig. 280 a), the cavities in which communicate with one another. The last of these passes freely into the medullary tube behind it. These primi- A tive cerebral vesicles give rise to new seg- ments, so that we can soon distinguish five. The first is known as the Fore-brain or Prosencephalon (Fig. 280, a); the next as the Twixt-brain or Thalamencephalon (/>); the Mid-brain or Mesencephalon (B C c) forms a third swelling; and this is succeeded by the Hind-brain or Metencephalon (d), and the After-brain or Myelencephalon (e), which is directly continuous with the spinal chord, and with the metencephalon. The metencephalon forms the most anterior por- tion of the roof of the myelencephalon, and is not therefore as distinct as the rest of the cerebral vesicles. At first, the vesicles are placed one behind the other, and lie in the line of the longitudinal axis of the spinal chord, bat they soon come to be set at an angle to one another. This is due to the unequal growth of the upper and lower portions, for the upper ones increase greatly in size. Those parts which are least developed become covered over by the growth of some of the upper parts. Between the prosencephalon and thalamen- cephalon the wall is thinned out, and a fissure- like portion developed (primitive cerebral cleft, Fig. 280, a), into the interior of which a process from the envelopes of the brain is
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
