. Anatomy, descriptive and applied. Anatomy. THE BRAIN OB ENCEPHALON 855 sagittally placed, slit-like form of the so-called third ventricle. The cavities of the optic and olfactory buds become obliterated. II. Mid-brain.—The second primary vesicle becomes .somewhat later differen- tiated and takes a less prominent part in the adult brain. Its dorsal wall goes into the formation of four eminences, the corpora quadrigemina, while the lateral and ventral sections grow considerably to form the crura cerebri. The neural cavity within the mid-brain persists as the narrow aqueduct joining the third a
. Anatomy, descriptive and applied. Anatomy. THE BRAIN OB ENCEPHALON 855 sagittally placed, slit-like form of the so-called third ventricle. The cavities of the optic and olfactory buds become obliterated. II. Mid-brain.—The second primary vesicle becomes .somewhat later differen- tiated and takes a less prominent part in the adult brain. Its dorsal wall goes into the formation of four eminences, the corpora quadrigemina, while the lateral and ventral sections grow considerably to form the crura cerebri. The neural cavity within the mid-brain persists as the narrow aqueduct joining the third and fourth ventricles. III. Hind-brain.—^The third primary brain vesicle is demarcated from the mid- brain by a marked constriction to which has been given the term isthmus rhomb- encephali. The hind-brain is specially characterized by the great expansion of its thinned-out, membranous dorsal wall caudad, while cephalad the dorsal wall becomes very much thickened as the proton or " anlage " of the cerebellum. The ventral and lateral parts undergo thickening to form the pons and medulla oblongata. Invaglnatioti of Ectoderm to form the tens ntdimenf. Fig, 626.—Trans-section of head of chick embryo of forty-eight hours' incubation. X 55. (From Duval's Atlas d'Embryologie.) Flexures of the Brain Tube.—The difference in growth rate of the differ- ent parts of the brain tube and the marked disproportion between the rapid brain growth and slower head growth causes the encephalic neural tube to become sharply bent upon itself at certain points. The first flexure to occur is involved in a bending of the entire head and takes place in the region of the mid-brain; this flexure is termed the cephalic flexure. A second bending of the tube occurs at the junction of the spinal cord and hind-brain; this is termed the cervical flexure, and is so pronounced in the fifth week of intrauterine life that the lirain tube and spinal cord form a right angle with each other. A third fle
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1913
