. Cunningham's Text-book of anatomy. Anatomy. HYPOTHALAMUS 4? *& o<? t Upper limit of rhombencephalon Recessus / infundibuli' Recessus mamillaris' Cephalic flexure' Even at a time, during the second week, when the anterior (oral) end of the neural tube is still open (neuroporus anterior), a right-angled bend has already developed in the rudiment of the brain (cerebral vesicle). Slightly less than half of the length of the vesicle had projected beyond the upper (anterior) end of the ..,...,.„ , , no tochord and became flexed Anterior limit of mesencephalon ,, n -,_,. ventrally round it (
. Cunningham's Text-book of anatomy. Anatomy. HYPOTHALAMUS 4? *& o<? t Upper limit of rhombencephalon Recessus / infundibuli' Recessus mamillaris' Cephalic flexure' Even at a time, during the second week, when the anterior (oral) end of the neural tube is still open (neuroporus anterior), a right-angled bend has already developed in the rudiment of the brain (cerebral vesicle). Slightly less than half of the length of the vesicle had projected beyond the upper (anterior) end of the ..,...,.„ , , no tochord and became flexed Anterior limit of mesencephalon ,, n -,_,. ventrally round it (-big. 455). ,/ This bend is known as the cephalic flexure. The region of the brain vesicle in which it develops will later on become the mesen- cephalon or mid-brain; and even at the early stage of development now under consideration (Eig. 455) there is a slight narrowing of the tube (isthmus) that marks the boundary be- tween the mid-brain and the rhombencephalon or hind-brain. Just beyond the end of the notochord there is an even fainter trace of a constriction in- dicating the line of de- marcation between the mid-brain and the prosen- cephalon or fore-brain. Shortly after the appearance of the cephalic flexure a similar bending occurs in the region where the encephalon becomes continuous with the medulla spinalis (Fig. 456, A). This is the cervical flexure. But at this stage, or even earlier (Fig. 456), there has been developing a third bend which produces effects differing from those just mentioned. At the end of the second week a slight bulging can be detected on the ventral side of the hind- RHOMB- 1NCEPHALON Notochord — Fig. 455. Upper limit of ' spinal medulla -Left Lateral Aspect of an Early Human Embryo (after His's model, reversed). CEPHALIC rLl tf\£-e» OPTIC VESICLE. A B Fig. 456.—Two Stages in the Development of the Human Brain (after His). A, Brain of an embryo of the third week. B, Brain of an embryo of five weeks. brain (Fig. 455): during the next four
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1914
