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| | | #REDIRECT [[Paper - The developmental alterations in the vascular system of the brain of the human embryo (1921)]] |
| :'''Links:''' [[Book_-_Contributions_to_Embryology|Carnegie Institution of Washington - Contributions to Embryology]]
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| ==Introduction==
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| One of the most striking features in the development of the blood-vessels of
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| the head is the clear way in which they demonstrate the embryological principle
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| of what may be termed integrative development. It is quite evident that the
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| vascular apparatus does not independently and by itself "unfold" into the adult
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| pattern. On the contrary, it reacts continuously in a most sensitive way to the
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| factors of its environment, the pattern in the adult being the result of the sum
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| of the environmental influences that have played upon it throughout the embryonic period. We thus find that this apparatus is continuously adequate and
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| complete for the structures as they exist at any particular stage; as the environmental structures progressively change, the vascular apparatus also changes and
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| thereby is always adapted to the newer conditions. Furthermore, there are no
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| apparent ulterior preparations at any time for the supply and drainage of other
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| structures which have not yet made their appearance. For each stage it is an
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| efficient and complete going-mechanism, apparently uninfluenced by the nature of
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| its subsequent morphology.
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| With these factors in mind, one can better understand the architectural
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| arrangements of the vascular system of the head that ai^pear in different periods of
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| development. In the primordial or precirculatory period the vessels that then
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| exist are engaged principally in growth and in the elaboration of a plexus, and
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| their form then is little influenced by conditions that would favor the circulation
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| of the contained fluid. As the circulatory flow of the blood becomes established
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| we find that the vascular plexus responds by conforming to the hydrodynamic
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| requirements, becoming adequately adapted to the form of the neural tube as then
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| existing, with favorably situated aortic feeders and simple and direct drainage
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| channels. As the brain becomes more complicated, and as the skull-membranes
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| form, there occur, step by step, the necessary adaptations on the part of the bloodvessels. Finally, when the permanent form is attained, the vessels lose their transi-
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| tory character and develop permanent and more highly differentiated walls, properly
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| suited to the adult functional requirements.
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| It is possible to subdivide the development of the blood-vessels of the brain
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| into five successive periods, each showing special adaptations to their changing
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| environmental conditions. To faciUtate the description of this process an arbitrary
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| order of that kind will be adopted in this paper. During the first of these five
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| periods there are established the primordial endothehal blood-containing channels,
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| which are neithei arteries nor veins, but constitute the source from which all the
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| arteries, veins, and capillaries of the brain are derived. These primordial bloodvessels lie bilaterally close along the brain-wall, at first either in the form of a
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| single, slender, longitudinal tube, as seen along the hindbrain, or a plexiform space
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| as seen near the forebrain and midbrain. Soon after the primordial vessels are
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| estabUshod, endothelial buds sprout from their walls and in conjuurtion with them
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| form an irregular endothelial vascular mcshwork which tends to spread over the
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| surface of the brain-wall, especially in the region of the forebrain and midbrain.
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| There is thus formed a plexiform system which constitutes a germinal bed of endo-
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| thelium rather than a circulatory apparatus.
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| During the second developmental period the primordial blood-vessel plexus
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| of the head slowly resolves itself into veins, arteries, and capillaries, and becomes
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| architecturally suited to the circulatory flow of the blood. That portion of the plexus which lies against the brain spreads out as a flattened capillary sheet, conforming everj'where to the shape of the brain-wall and its attached ganglia and
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| sense-organs. The more superficial part of the plexus develops a coarser mesh and
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| forms larger channels, which tend to unite into continuous trunks and gradually,
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| by virtue of their commimications, can he recognized as definite arteries and veins.
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| The intermediate loojis of the plexus maintain the anastomosis between the deep
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| capillary sheet and the more superficial trunks, forming tributaries of the veins
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| and branches of the arteries. The second period thus establishes the primary
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| type of the circulation of the head, in which there is a capillar}' bed, fed by arterial
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| branches from the aortic system and drained bilaterally by a continuous venous
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| trunk which extends back to the venous end of the heart.
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| The third period is inaugurated by the differentiation of the membranous
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| skull, the dura mater, and the arachnoid-pial membranes. As a result of this stratification of the tissues of the head, the more ventral of the anastomosing channels
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| that connect the deep capillary plexus with the superficial vessels l^ecome closed
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| off and there is a general separation or cleavage of the vessels immediately surrounding the brain-wall from those belonging to the membranous skull and its
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| coverings. This ]H-ocess begins at the base of the skull and extends bilaterally
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| upward toward the middle line of the vault, in which region the communications
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| between the deeper and more superficial systems are to some extent maintained.
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| In this way the cerebral vessels are gradually separated from the dural vessels and
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| in a similar manner the superficial vessels of the head become isolated by the laying
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| down of the ])rimordium of the membranous skull, after which their course of
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| development is quite indejjendent of that of the dural and cerebral systems.
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| By the time the third i)eri()d is well under way it is overlapped by the fourth
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| period, under which we include the remarkable series of adjustments in the arrangement of the blood-vessels, in adaptation to the develoi^mental alterations in the
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| form, size, and condition of the structures of the head region. The brain is one of
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| the chief factors in this process. The marked change in its form, and especially
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| the prolonged relative growth of the cerebral hemispheres, render necessary a
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| continuous series of alterations in the blood-channels that extend far into the late
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| fetal stages. In the earlier stages a fundamental change results from the growth
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| of the labyrinth and its cartilaginous capsule, whereby a mechanical oltstruction is introduced that results in the obhteration of a considerable part of the largest vein
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| of the head. This is compensated for by a new channel which takes a more dorsal
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| course, and which eventually forms the sigmoid portion of the lateral sinus.
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| Finally, under the fifth period we would include the late histological changes in
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| the walls of the vessels that convert them into the adult arteries, veins, and the
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| various types of sinuses. These histological factors, however, are not considered
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| in the present paper and are merely mentioned to complete the sequence, as they
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| have no determining influence on the phenomena of the preceding four periods.
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| The observations that are reported are chiefly concerned with the third and
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| fourth developmental periods; that is, after the primary circulation of the head is
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| established and during the cleavage and adjustmental stages. A review, however,
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| will be made of the liistory of the vascular system of the head previous to that
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| time, which will be based in large part on the important observations of Evans
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| (1909, 1912) and Pabin (1915, 1917o, 19176). The study of the development of
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| the vascular system of the head of the human embryo was initiated in this laboratory
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| by Professor Mall (1905). Later I continued the same investigation and reported
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| (Streeter, 1915) some of the features of the adaptive metamorphosis of the dural | |
| veins. In the present paper I shall include much of the same subject-matter and
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| incorporate with it further observations made on additional material, thus making
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| it possible to treat the general subject more completely and to demonstrate the
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| relations of the dural veins to the principal arteries of the head.
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| -----Content to be added----
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| {{Template:Historic Disclaimer}}
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| {{Glossary}}
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| {{Footer}}
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| [[Category:Historic Embryology]] [[Category:Human]] [[Category:Cardiovascular]]
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