Book - Quain's Embryology 6 - Revision history

Z8600021 at 11:43, 14 March 2015

2015-03-14T11:43:36Z

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	Fig. 546. — Transverse Section THROUGH THE EjlBRYO OF THE ChICK, AND		Fig. 546. — Transverse Section THROUGH THE EjlBRYO OF THE ChICK, AND

	A, Blastoderm at the end		A, Blastoderm at the end of the First Day. Magnified FROM 90 TO 100 TIMES (from Kolliker).

	~~Of THE~~ First Day. Magnified FROM 90 TO 100 TIMES (from Kolliker).

	d'/> tUuh d d i,a-p c/i /i, epiblast ; (?c?, hypoblast ;		d'/> tUuh d d i,a-p c/i /i, epiblast ; (?c?, hypoblast ;
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	In birds and mammals there is no distinction to be seen at first between the outer or corneous layer of the involuted epiblast and the cells which by their increase more immediately constitute the medullary plates. In batrachia, however, the dark colour of the corneous layer shows it to be distinct from the more strictly nervous layers. In osseous fishes there is no medullary groove or canal at first, but an involution of a solid column of cells, which is subsequently hollowed out for the formation of a ventricular cavity.		In birds and mammals there is no distinction to be seen at first between the outer or corneous layer of the involuted epiblast and the cells which by their increase more immediately constitute the medullary plates. In batrachia, however, the dark colour of the corneous layer shows it to be distinct from the more strictly nervous layers. In osseous fishes there is no medullary groove or canal at first, but an involution of a solid column of cells, which is subsequently hollowed out for the formation of a ventricular cavity.

	The masses of grey matter first formed in the spinal marrow correspond chiefly to the anterior columns ; these are succeeded by lateral masses or columns, and somewhat later by small posterior columns. There are at first no commissures except by the passage of the deepest		The masses of grey matter first formed in the spinal marrow correspond chiefly to the anterior columns ; these are succeeded by lateral masses or columns, and somewhat later by small posterior columns. There are at first no commissures except by the passage of the deepest layer of cells across the middle line, but the fibres from the roots of the nerves when formed are traceable into the grey substance of their rcs])cctive anterior and posterior columns.


	Fig. 548. — Transverse Section of the CervicaIi Part of the Spinal Cord of a Human Ejibryoop Six Weeks (from Kolliker). 'f		Fig. 548. — Transverse Section of the CervicaIi Part of the Spinal Cord of a Human Ejibryoop Six Weeks (from Kolliker). 'f
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	Tins and the following figure are only sketched, tJie white matter and a part of the gre.y not being shaded in. r, central canal ; e, its epithelial lining, at e (inferiorly), the i)art which becomes the anterior comniissure ; at c (superiorly), the original place of closure of the canal ; a, the white substance of the anterior columns, beginning to be separated from the grey matter of the interior, and extending round into the lateral column, where it is crossed by the line from [/, which points to the grey substance ; p, posterior column ; ar, anterior roots ; j5r, posterior roots.		Tins and the following figure are only sketched, tJie white matter and a part of the gre.y not being shaded in. r, central canal ; e, its epithelial lining, at e (inferiorly), the i)art which becomes the anterior comniissure ; at c (superiorly), the original place of closure of the canal ; a, the white substance of the anterior columns, beginning to be separated from the grey matter of the interior, and extending round into the lateral column, where it is crossed by the line from [/, which points to the grey substance ; p, posterior column ; ar, anterior roots ; j5r, posterior roots.



	~~layer of cells across the middle line, but the fibres from the roots of the nerves when formed are traceable into the grey substance of their rcs])cctive anterior and posterior columns.~~

	The white substance is formed external to or on the surface of the deeper grey substance j but it is not yet determined whether it is developed out of the cells composing the grey matter or from separate blastema to which the mesoblast may in part contribute. It is combined with connective tissue elements, and its structure is <lifferent from that of the grey substance, which is undoubtedly produced by multiplication and differentiation of the involuted epiblastic cells.		The white substance is formed external to or on the surface of the deeper grey substance j but it is not yet determined whether it is developed out of the cells composing the grey matter or from separate blastema to which the mesoblast may in part contribute. It is combined with connective tissue elements, and its structure is <lifferent from that of the grey substance, which is undoubtedly produced by multiplication and differentiation of the involuted epiblastic cells.
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	~~THE BRAIN OR ENCEPHALON.~~		==The Brain 0r Encephalo==

	1. — General phenomena of development as ascertained in birds and mammals. — A reference has previously been made to the simple form in which the brain at first presents itself in the anterior dilated portion of the primitive medullary tube, and its partial division into the three primary cerebral vesicles. This is placed within simple cranial walls formed by the cephalic inflection of the blastoderm, without face or any other parts ; so that the head of the embryo consists at first of no more than the wider part of the medullary tube and the simple enclosing wall.		1. — General phenomena of development as ascertained in birds and mammals. — A reference has previously been made to the simple form in which the brain at first presents itself in the anterior dilated portion of the primitive medullary tube, and its partial division into the three primary cerebral vesicles. This is placed within simple cranial walls formed by the cephalic inflection of the blastoderm, without face or any other parts ; so that the head of the embryo consists at first of no more than the wider part of the medullary tube and the simple enclosing wall.
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	1. The cerebral hemispheres, with their ventricular hollows or lateral ventricles, the corpora striata, and the olfactory lobes, — a set of parts to which, as a whole, the name of proceredrum or j)rosejice2}haIo?i may be Q-iveu.		1. The cerebral hemispheres, with their ventricular hollows or lateral ventricles, the corpora striata, and the olfactory lobes, — a set of parts to which, as a whole, the name of proceredrum or j)rosejice2}haIo?i may be Q-iveu.




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	III. Posterior primary Vesicle,		III. Posterior primary Vesicle,

			Cerebellum, Pons Varolii, anterior part of f- 4. Eiicncephalon. \ the Fom-th Ventricle.

	< Cerebellum, Pons Varolii, anterior part of f- 4. Eiicncephalon. \ the Fom-th Ventricle.

	"j ( Medulla Oblongata, Fourth Ventricle, Au		"j ( Medulla Oblongata, Fourth Ventricle, Au
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	F, frontal lohe ; P, parietal ; 0, occipital ; T, temporal ; I, olfactory bulb ; II, right optic nerve ; f p, fronto-parietal fissure ; p, vertical fissure ; ;/, internal vertical fissui'e ; /(, hippocampal fissure ; ff, gyrus formicatus ; c, c, corpxis callosum ; s, septum lucidum ; /, placed between the middle commissure and the foramen of Monro ; V, in the upper part of the third ventricle immediately below the velum interpositum ami fornix ; v', in the back part of the third ventricle below the pineal gland, and pointing l)y a line to the aqueduct of Sylvius ; v" , in the lower part of the third ventricle above the infundibuhim ; )•, recessus pinealis passing backwards from the tela choroidea ; p r,pons Varolii ; C c, cerebellum.		F, frontal lohe ; P, parietal ; 0, occipital ; T, temporal ; I, olfactory bulb ; II, right optic nerve ; f p, fronto-parietal fissure ; p, vertical fissure ; ;/, internal vertical fissui'e ; /(, hippocampal fissure ; ff, gyrus formicatus ; c, c, corpxis callosum ; s, septum lucidum ; /, placed between the middle commissure and the foramen of Monro ; V, in the upper part of the third ventricle immediately below the velum interpositum ami fornix ; v', in the back part of the third ventricle below the pineal gland, and pointing l)y a line to the aqueduct of Sylvius ; v" , in the lower part of the third ventricle above the infundibuhim ; )•, recessus pinealis passing backwards from the tela choroidea ; p r,pons Varolii ; C c, cerebellum.

Z8600021: /* Prosencephalon */

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Prosencephalon

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	Each hemispherevesicle becomes divisible into two parts : one of these is the part which from the interior appears as the corpus striatum, and from the exterior as the island of Eeil, or central lobe ; the other fonns the expanded or covering portion of the hemisphere, and is designated by Reichert the mantle. The aperture existing at the constricted neck of the hemispherevesicle, Schmidt and Reichert have recognised as the foramen of Monro.		Each hemispherevesicle becomes divisible into two parts : one of these is the part which from the interior appears as the corpus striatum, and from the exterior as the island of Eeil, or central lobe ; the other fonns the expanded or covering portion of the hemisphere, and is designated by Reichert the mantle. The aperture existing at the constricted neck of the hemispherevesicle, Schmidt and Reichert have recognised as the foramen of Monro.

	The corpora striata, it will be observed, have a different origin from the optic thalami ; for, while the latter are formed by thickening of the cu-cumferential wall of a part of the first cerebral vesicle, and thus correspond in their origin with all the parts of the encephalon behind them, which are likewise derived from portions of the cerebro-spinal tube, the corpora striata appear as thickenings of the floor of the hemisphere-vesicles, which are lateral ofl:'-shoots from the original cerebro-spinal tube. On this account, Reichert considers the brain primarily divisible into the stem, which comprises the whole encephalon forwards to the taenia semicircularis, and the hemisphere-vesicles, which include the corpora striata and hemispheres.

			The corpora striata, it will be observed, have a different origin from the optic thalami ; for, while the latter are formed by thickening of the cu-cumferential wall of a part of the first cerebral vesicle, and thus correspond in their origin with all the parts of the encephalon behind them, which are likewise derived from portions of the cerebro-spinal tube, the corpora striata appear as thickenings of the floor of the hemisphere-vesicles, which are lateral off-shoots from the original cerebro-spinal tube. On this account, Reichert considers the brain primarily divisible into the stem, which comprises the whole encephalon forwards to the taenia semicircularis, and the hemisphere-vesicles, which include the corpora striata and hemispheres.


	Fig. 561. — View of the Inner Surface of the Right Half ~~OF THE~~ Foetal ~~BrAIN OF ABOUT SIX MONTHS~~ (from Reichert).
			Fig. 561. — View of the Inner Surface of the Right Half of the Foetal Brain of about Six Months. (from Reichert).

	F, frontal lohe ; P, parietal ; 0, occipital ; T, temporal ; I, olfactory bulb ; II, right optic nerve ; f p, fronto-parietal fissure ; p, vertical fissure ; ;/, internal vertical fissui'e ; /(, hippocampal fissure ; ff, gyrus formicatus ; c, c, corpxis callosum ; s, septum lucidum ; /, placed between the middle commissure and the foramen of Monro ; V, in the upper part of the third ventricle immediately below the velum interpositum ami fornix ; v', in the back part of the third ventricle below the pineal gland, and pointing l)y a line to the aqueduct of Sylvius ; v" , in the lower part of the third ventricle above the infundibuhim ; )•, recessus pinealis passing backwards from the tela choroidea ; p r,pons Varolii ; C c, cerebellum.		F, frontal lohe ; P, parietal ; 0, occipital ; T, temporal ; I, olfactory bulb ; II, right optic nerve ; f p, fronto-parietal fissure ; p, vertical fissure ; ;/, internal vertical fissui'e ; /(, hippocampal fissure ; ff, gyrus formicatus ; c, c, corpxis callosum ; s, septum lucidum ; /, placed between the middle commissure and the foramen of Monro ; V, in the upper part of the third ventricle immediately below the velum interpositum ami fornix ; v', in the back part of the third ventricle below the pineal gland, and pointing l)y a line to the aqueduct of Sylvius ; v" , in the lower part of the third ventricle above the infundibuhim ; )•, recessus pinealis passing backwards from the tela choroidea ; p r,pons Varolii ; C c, cerebellum.
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	The cerebral hemispheres enlarging, and having their walls increased in thickness, fonn, dming the fourth month (Tiedemann), two smooth shell-like ~~lamellfe~~. which include the cavities afterwards named the lateral ventricles, and the parts contained within them. Following out the subsequent changes affecting the exterior of the cerebral hemispheres, it is found that about the foiuth. month the first traces of some of the conrolvtions appear, the intermediate ~~svlci~~ commencing- only as very slight depressions on the hitherto smooth sm-face. Though the hemispheres continue to grow quickly upwards and backwards, the convolutions at first become distinct by comparatively slow degrees ; but towards the seventh and eighth months they are developed with gi-eat rapidity, and at the beginning of the last month of intra-uterine life, all the principal ones are marked out.		The cerebral hemispheres enlarging, and having their walls increased in thickness, fonn, dming the fourth month (Tiedemann), two smooth shell-like lamellae. which include the cavities afterwards named the lateral ventricles, and the parts contained within them. Following out the subsequent changes affecting the exterior of the cerebral hemispheres, it is found that about the foiuth. month the first traces of some of the conrolvtions appear, the intermediate sulci commencing- only as very slight depressions on the hitherto smooth sm-face. Though the hemispheres continue to grow quickly upwards and backwards, the convolutions at first become distinct by comparatively slow degrees ; but towards the seventh and eighth months they are developed with gi-eat rapidity, and at the beginning of the last month of intra-uterine life, all the principal ones are marked out.


			The Sf/!r!fi)i _p\\:finv, which afterwards separates the anterior from the middle lobe of each hemisphere, begins as a depression or cleft between them about the fourth month, and, after the great longitudmal, is the first of the fissures to make its appearance. It is followed by the fissure of Rolando, and the vertical or parieto-occipital fissure, and somewhat later by the internal fronto-parietal fissui-e. After this, the various subordinate fissures dividing the convolutions gradually make their appearance. By the end of the third month the hemispheres have extended so far backwards as to cover the thalami ; at the fourth they reach the corpora quadrigemina ; at the sixth they cover those bodies and great part of the cerebellum, beyond which they project still fiuther backwards by the end of the seventh month.

	The Sf/!r!fi)i _p\\:finv, which afterwards separates the anterior from the middle lobe of each hemisphere, begins as a depression or cleft between them about the fourth month, and, after the great longitudmal, is the first of the fissures to make its ajipearance. It is followed by the fissure of Rolando, and the vertical or parieto-occipital fissure, and somewhat later by the internal fronto-parietal fissui-e. After this, the various subordinate fissures dividing the convolutions gradually make their appearance. By the end of the third month the hemispheres have extended so far backwards as to cover the thalami ; at the fourth they reach the corpora quadrigemina ; at the sixth they cover those bodies and great part of the cerebellum, beyond which they project still fiuther backwards by the end of the seventh month.

	During the growth of the hemisphere the aperture of the foramen of Monro is extended backwards ; the arched margin of this opening is curved downwards at its extremities, and forms anteriorly the fornix, and posteriorly the corpus fimbriatum and hippocampus major ; above the margin a part of the wall of each hemisphere comes into contact with its fellow, and in the lower part forais the septum lucidum, while above this the hemispheres are united by the development of the great commissure, the coipus callosum.		During the growth of the hemisphere the aperture of the foramen of Monro is extended backwards ; the arched margin of this opening is curved downwards at its extremities, and forms anteriorly the fornix, and posteriorly the corpus fimbriatum and hippocampus major ; above the margin a part of the wall of each hemisphere comes into contact with its fellow, and in the lower part forais the septum lucidum, while above this the hemispheres are united by the development of the great commissure, the coipus callosum.

	The corpus ~~callo.sum~~ is described by Tiedemann as being first seen about the end of the third month, as a narrow vertical band, extending across between the forepart of the two hemispheres, and subsequently growing backwards. With this view the observations of Schmidt coincide. Reichert, however, maintains that the commissural structure seen at the forepart of the hemispheres is the anterior white commissure, and that the coiiaus callosum appears in its whole extent at once.
			The corpus callosum is described by Tiedemann as being first seen about the end of the third month, as a narrow vertical band, extending across between the forepart of the two hemispheres, and subsequently growing backwards. With this view the observations of Schmidt coincide. Reichert, however, maintains that the commissural structure seen at the forepart of the hemispheres is the anterior white commissure, and that the coiiaus callosum appears in its whole extent at once.


	The corpora alhtcantui at fixst form a single mass : so also do the anterior pillars of the fornix, which are distinguished before the posterior pillars. The posterior pillars are not seen until the fourth or fifth month. At that period the hippocampus minor is also discernible.		The corpora alhtcantui at fixst form a single mass : so also do the anterior pillars of the fornix, which are distinguished before the posterior pillars. The posterior pillars are not seen until the fourth or fifth month. At that period the hippocampus minor is also discernible.

	~~DEVELOPMENT OF THE NERVES.~~

	==Nerves==		==Nerves==

Z8600021: /* Spinal Nerves */

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Spinal Nerves

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	DEVELOPMENT OF THE NERVES.		DEVELOPMENT OF THE NERVES.

			==Nerves==
	===Spinal Nerves===		===Spinal Nerves===

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2012-10-11T22:32:47Z

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Z8600021 at 22:24, 11 October 2012

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			=Development of the Nervous System=


			{{Quain header}}


	~~==Development of the Nervous System==~~

	===The Cerebro-spinal Centre===		===The Cerebro-spinal Centre===
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	Observations are still wanting to determine whether the olfactory nerves are developed from the bulbs, and have thus a cerebral origin, or are separately formed from peripheral blastema like all other nerves, with the exception of the optic.		Observations are still wanting to determine whether the olfactory nerves are developed from the bulbs, and have thus a cerebral origin, or are separately formed from peripheral blastema like all other nerves, with the exception of the optic.

			{{Quain footer}}

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2012-10-11T08:34:16Z

Created page with " ==Development of the Nervous System== ===The Cerebro-spinal Centre=== From what has been previously stated it will have been seen that the rudiment of the cerebro-spinal..."

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