Paper - The development of the gyri and sulci on the surface of the island of Reil of the human brain (1891)
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Cunningham DJ. The development of the gyri and sulci on the surface of the island of Reil of the human brain. (1891) J Anat. Physiol. 25: 338-348. PMID 17231925
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The Development of the Gyri and Sulci on the Surface of the Island of Reil of the Human Brain
D. J. Cunningham, M.D., Professor of Anatomy, University of Dublin.
- This is an abstract of a portion of a Memoir on the Cerebral Surface, which is in course of preparation. Cunningham Memoir, No. VII., R.2.A.
The admirable description of the gyri and sulci on the surface of the island of Reil, which has been given by Eberstaller, leaves little to be desired in so far as the adult brain is concerned. He points out that the insula is divided into an anterior and a posterior part by a sulcus which lies in the same plane and presents the same direction as the fissure of Rolando. This furrow had previously been noted by Hefftler? and Guldberg,* and the latter author has suggested for it the very appropriate name of sulcus centralis insule, which indicates not only its central position in the island of Reil, but also its relation to the central fissure on the outer face of the hemisphere mantle. The portions of the island of Reil which lie in front of and behind this sulcus, Eberstaller has termed the insula anterior and the insula posterior.
It will be necessary for me to state briefly the further points in the anatomy of the island of Reil which have been elucidated by Eberstaller, in order that I may be able to render intelligible the few additions to his description which I wish to make, as well as the facts relating to the development of the convolutions and sulci of the insula which I have observed. According to Eberstaller the anterior insula is connected entirely with the frontal lobe, whilst the posterior insula is exclusively connected with the parietal and temporal lobes.
The insula anterior presents three gyri which unite below to form the pole of the island of Reil, whilst above they are separated from each other by two sulci. These three convolutions Eberstaller names from before backwards, the gyrus brevis primus, the gyrus brevis secundus, and the gyrus brevis tertius. The gyrus primus and the gyrus tertius are, as a rule, strongly marked, whilst the intermediate one is more weakly developed. It appears to me that a better name for the gyrus tertius would be the gyrus centralis anterior, seeing that this term would indicate its position with reference to the central sulcus, and at the same time show its relation to the anterior central (or ascending frontal) convolution on the outer face of the hemisphere. The gyrus tertius is continued downwards on the surface of the island ‘of Reil very much in the direction of the ascending frontal convolution, and its upper end lies concealed under that part of the fronto-parietal operculum which is formed by this convolution of the frontal lobe. Of the two sulci which separate the three gyri breves, the anterior, termed by Eberstaller the sulcus anterior, is always well expressed, but it rarely reaches so low as the pole of the fore island. For reasons which will become more apparent afterwards, the term sulcus precentralis Reilii, proposed by Guldberg, would be more appropriate for this sulcus. As a rule, the second sulcus is often little more than a shallow depression of a triangular form which intervenes between the upper portions of the gyrus secundus and the gyrus tertius.
2 “Zur Anatomie und Morphologie der Insula Reilii,” Anatomischer Anzeiger, No. 24, 15th November 1887, p. 739.
3 Vide a Report upon Dr Hefftler’s Inaugural Dissertation upon ‘‘ Die Grosshirnwindungen des Menschen und deren Bezeichungen zum Schiadeldach,” by Prof. Landzert, in the Archiv. fiir Anthropologie, Band x., 1878.
4 “Zur Morphologie der Insula Reilii,” Anatomischer Anzeiger, No. 21, Ist October 1887, p. 659.
But Eberstaller describes two additional gyri in connection with the insula anterior. These he terms the gyrus transversus and gyrus accessorius. The gyrus transversus extends forwards from the pole of the island, and is of the nature of an annectant gyrus, seeing that it connects the lower part of the insula anterior with the under or orbital face of the frontal lobe. When superficial, as indeed it very frequently is, it is interposed as a barrier between the lower end of the furrow which limits the island of Reil in front and the vallecula Sylvii. Sometimes however, it is depressed and deep, and then a superficial connection occurs between the anterior limiting furrow of the insula and the vallecula.
The gyrus accessorius is placed on the outer side of the gyrus transversus. It stretches forwards from the fore part of the gyrus brevis primus, and lies under cover of the orbital operculum at a lower level than the anterior horizontal limb of the Sylvian fissure. As Eberstaller has pointed out, this small convolution is frequently carried right across the anterior limiting sulcus of the insula, and becomes continuous with a corresponding gyrus on the deep surface of the outer part of the orbital operculum. Eberstaller lays great stress upon the connection which he believes to exist between the gyrus accessorius and the external orbital limb of the Sylvian fissure. He states that, in cases where this limb is absent, there is a corresponding furrow on the deep surface of the orbital operculum which exactly overlies the gyrus accessorius. I would point out, however, that this furrow should be considered as being more in relation to the sulcus on the surface of the insula which bounds the gyrus accessorius on its mesial aspect. When the gyrus in question is carried across the anterior limiting sulcus it joins that part of the orbital operculum which lies between the anterior horizontal and the external orbital limbs of the Sylvian fissure.
The insula posterior is divided into two convolutions, which lie one in front of the other, by a well-marked sulcus, which may be called the sulcus postcentralis Reilii. This furrow extends upwards and backwards into the posterior part of the superior limiting furrow of the island of Reil, but, in front and below, the two convolutions unite beyond it. For the anterior convolution, which is covered at its upper end by that part of the frontoparietal operculum which is formed by the base of the ascending parietal convolution, Eberstaller employs the term suggested by Giacomini, viz., the gyrus longus; but it would be much better named the posterior central gyrus,—a term which would not only indicate its position with reference to the sulcus centralis, but also its relations to the posterior central convolution on the outer face of the hemisphere. The posterior convolution of the hinder insula Eberstaller terms the gyrus posterior secundus. The upper end of this convolution stands in relation to the fore part of the supramarginal convolution of the parietal lobe.
As we have seen, the two gyri of the posterior insula unite below the postcentral sulcus, and are then continued onwards by a common stem. This is separated from the pole of the anterior insula by the sulcus centralis, and Eberstaller considers that it is carried on to the deep surface of the extremity of the temporal lobe, on which, he states,it can be traced to the temporal pole. Certainly in the adult the condition of affairs presents this appearance, but we shall see later on that a study of the development of the parts appears to indicate that the connection is not with the temporal lobe, but with the great limbic lobe.
There is thus a very cloge correspondence between the convolutions and sulci on the surface of the island of Reil and those on the lateral surface of the hemisphere. The two central convolutions (7.¢., gyrus brevis tertius and gyrus longus of Eberstaller) correspond with the two central convolutions (ascending frontal and ascending parietal), and the three fissures, viz., the sulcus precentralis Reilii, the sulcus centralis Reilii, and the sulcus postcentralis Reilii, are in every respect comparable with the sulcus precentralis inferior, the fissure of Rolando, and the vertical limb of the intraparietal fissure on the surface of the mantle. It is true that we cannot regard these corresponding convolutions and sulci as being directly continuous with each other, but still, in many cases, something which approaches very nearly to continuity occurs. Thus it is well known that the inferior preecentral sulcus, the fissure of Rolando, and the intraparietal suleus are not infrequently carried downwards, so as to cut into the fronto-parietal operculum, and open into the Sylvian fissure. Eberstaller has pointed out that when this occurs in the case of the fissure of Rolando the condition is brought about by the extension upwards round the lower margin of the operculum of a small variable furrow, which he terms the inferior transverse sulcus. This undoubtedly belongs to the same system as the fissure of Rolando and the sulcus centralis Reilii—min other words, it is to be regarded as the connecting link. The extension of the intraparietal into the fissure of Sylvius is effected in precisely the same manner. A small variable opercular furrow extends upwards into it, and acts as a loose bond of union between it and the postcentral sulcus of the insula. I am not able to speak with the same certainty of the two precentral sulci. These two furrows, the one on the surface of the insula and the other on the surface of the mantle, do not maintain the same prominence which they present in the foetus, and consequently their relations in the adult cannot be studied with the same precision. At the same time, I may say that everything is in favour of the view that, in those cases in which the precentral furrow of the mantle opens into the Sylvian fissure, it does so by the development of an opercular furrow intermediate between it and the precentral sulcus of the insula. When we come to deal with the development of the sulci on the insula, we shall see that the precentral sulcus becomes slightly shifted in its position in a forward position, so that it does not accurately coincide in its direction in the adult with the corresponding furrow of the frontal lobe.
But, further, the inferior frontal convolution stands in close connection with the portion of the anterior insula which is placed in front of the precentral sulcus of the island of Reil. Thus the gyrus brevis primus, in its upper part, lies under cover of the pars triangularis, and not infrequently pushes itself across the upper limiting sulcus of Reil to form a direct connection with this part of the lower frontal convolution. The gyrus accessorius stands in precisely the same relation to that part of the orbital portion of the inferior frontal convolution which lies between the anterior horizontal limb of the Sylvian fissure and the posterior extremity of the external orbital sulcus. The connection between the sulcus which limits the gyrus accessorius below, and the external orbital sulcus on the orbital face of the frontal lobe, is also apparent. The former sulcus is continued on to the deep surface of the orbital operculum, and in certain cases cuts right through it to form the external orbital limb of the Sylvian fissure. Benedikt terms this limb the “ hinderpiece ” of the external orbital sulcus, and remarks that he has observed the two fissures to become continuous with each other. This condition I have never seen; but, be this as it may, I regard the external orbital limb of the Sylvian fissure in the same light as I do the inferior transverse sulcus of Eberstaller. It is a secondary fissure in the operculum, which is to be regarded as an intermediate piece between the sulcus on the insula which bounds the gyrus accessorius below and the external orbital sulcus on the orbital face of the frontal lobe.
But a very considerable amount of variation is to be noted in the arrangement of the gyri in the fore part of the insula, and it is of importance to observe that this goes hand in hand with variations in the corresponding part of the inferior frontal convolution. A disturbance in the condition of the one is generally accompanied by a disturbance in the condition of the other.
A study of the manner in which the convolutions and sulci of the insula are developed reveals many points of high importance and interest. Up to the middle of the fifth month the surface of the insula remains perfectly smooth, but long before this there is a marked indication of its division into a frontal and a parieto-limbic portion. From its very earliest appearance the fossa Sylvii is encroached upon below by the vallecula Sylvii. When the hemisphere is viewed in profile the latter region appears as a semilunar depression, which indents the lower part of the Sylvian fossa, and occupies a position between the extremity of the temporal lobe and the under aspect of the frontal lobe. This notch or depression is surrounded above by a rim which represents the external root of the olfactory lobe, and it divides the lower part of the Sylvian area into two nearly equal portions, viz., an anterior portion, which joins the under surface of the frontal lobe, and a posterior part, which runs into the extremity of the temporal lobe. As the temporal operculum begins to take shape, and the extremity of the temporal lobe grows forwards so as to overshadow the Sylvian vallecula, the hinder part of the lower portion of the insula becomes hidden from view, whilst the fore part develops into the pole of the insula. At the same time, the wide semilunar notch in the lower part of the insula, or, in other words, the outer extremity of the vallecula Sylvii, becomes gradually reduced in width, and converted into a narrow cleft or incision. Further, with the growth of the temporal operculum, the lower limiting furrow (lower part of the sulcus circularis Reilii of Schwalbe) becomes apparent.
When the front extremity of the temporal lobe of a foetal brain (say at the fifth month) is examined, two very distinct gyri, which traverse it from above downwards immediately external to the uncus, are evident. The outer of these is the extremity of the temporal operculum ; the inner gyrus is continuous with the hinder part of the insula, and lies close to the outer side of the front end of the uncus, which, as is well known, very early takes shape. These may be termed the primitive polar gyri of the temporal lobe. A faint furrow intervenes between them, but they are rendered more conspicuous by their own prominence than by the presence of the intervening sulcus. This sulcus is quite continuous with the posterior limiting sulcus of the insula, and when followed round the temporal pole it is seen to be directly in the line of the depression, along the bottom of which the collateral fissure is afterwards developed. Later it becomes a distinct and sharply-cut fissure, the incisura temporalis of Schwalbe. Coincident with this change, the inner of the two polar gyri, which is continuous with the hinder part of the insula, decreases in size and prominence until, in the eighth month, it becomes more or less completely incorporated with the uncus.
The inferior limiting sulcus of the insula, the incisura temporalis, which bounds the extremity of the uncus on its outer side, and the collateral fissure lie, therefore, all in the same line, and may be regarded as the bounding fissures of the temporal lobe. The inferior limiting sulcus of the insula marks it off from the island of Reil, while the incisura temporalis and the collateral fissure intervene between it and the great limbic lobe. But further, the growth forward of the extremity of the temporal pole is merely a growth of the anterior part of the temporal operculum, which, as we have seen, first shows as the outermost of the two primitive polar gyri. It extends forwards to meet the orbital operculum, and finally comes to overlap it to a very considerable extent. There is no part of the opercular arrangement which ultimately attains so great a depth.
Using the term “temporal pole,” therefore, in its more restricted sense (that is to say, excluding the extremity of the uncus), it is important to note that this part of the temporal lobe owes its existence entirely to the forward growth of the ’ operculum.
But in the later stages of development the very evident relation which the inferior limiting sulcus of the insula bears to the incisura temporalis and the collateral fissure becomes obscured. Towards the end of fetal life the deep surface of the temporal pole becomes scored with two or three transverse sulci. These have been figured and described in the adult brain with great care and exactness by Eberstaller.1 The same author has very correctly pointed out that in the adult the lower limiting sulcus of the insula ends in front ky running forwards upon the deep surface of the temporal pole, and taking up a position parallel to and behind these transverse sulci. This, however, is a purely adventitious arrangement, which is brought about through one of the transverse sulci, about the ninth month of foetal life, running into the limiting sulcus of Reil, and thus becoming continuous with it. In the adult brain, therefore, the true relations of the inferior limiting sulcus of the insula are somewhat misleading.
1 Anatomischer Anzeiger, No. 24, November 1887, p. 745.
The examination of the foetal brain renders the relationship which exists between the sulci and convolutions of the insula and the fissures and gyri on the surface of the mantle still more obvious. Three radial “ Primarfurche” appear in each region, not only at the same stage in the development of the brain, but also, as a rule, in very much the same order. In the latter half of the fifth month the central sulcus becomes evident as a faint linear furrow, which runs upwards and backwards from the lower part of the Sylvian fossa. From the very first it lies accurately in the line of the fissure of Rolando, and it appears at the same date. At this period the sulcus centralis is situated much nearer to the hinder end of the insula than in the later stages, because the Sylvian fossa has not yet attained its full degree of backward extension. In all the subsequent changes which occur in this region the sulcus centralis remains absolutely fixed and stationary, and sways neither in a forward nor in a backward direction.
The precentral sulcus is developed a little later than the central sulcus, but as a general rule before the end of the fifth month. It lies accurately in line with the sulcus precentralis inferior on the surface of the frontal lobe; but in its subsequent history it is not so stationary as the sulcus centralis. In the last four weeks of foetal life its upper end generally moves forwards to a slight extent, so that in a measure it loses its accurate relationship to the corresponding sulcus on the surface of the mantle. Another peculiarity of this sulcus consists in the fact that in the early stages of its development it generally outstrips the central sulcus, and for a time it becomes the best marked furrow on the surface of the insula. This pre-eminence it loses in the eighth month. In fact, as we have seen, it is in the adult the feeblest sulcus of the series. In connection with this it is of interest to note that very much the same thing frequently occurs in the case of the precentral fissure of the frontal lobe. In many cases, more especially when it appears earlier than the fissure of Rolando, the precentral sulcus of the mantle is extremely deep and much the most evident of the three radial “Primarfurche.” Later on, however, it falls behind the others in its degree of development. Pansch! held it as a law that there is a general correspondence between the depth of a furrow and its period of origin. In other words, the earlier a furrow makes its appearance in the foetal brain the deeper will it be in the adult brain in comparison with others of more recent development. This law is no doubt true in the main, but there are many exceptions, and the case in point is one of these.
Guldberg, in his excellent paper on the “ Morphology of the Island of Reil,” ? has mistaken in the foetal brain the sulcus precentralis for the sulcus centralis of the insula; and to account for its subsequent change in position, he has supposed that its upper end is gradually pushed backwards, until it assumes a situation and direction which brings it into a line with the fissure of Rolando. To account for this change of position, he is obliged to assume “that the frontal lobe of the cerebrum increases during growth relatively more than the part which is placed behind the sulcus centralis insule.” We know that this is not the case, because if it were we should also have the position of the fissure of Rolando affected; and this is almost as stationary and fixed after its first appearance as the sulcus centralis insule. But, further, the Sylvian fossa has a growth peculiar to itself. The anterior end of the fossa maintains throughout its entire growth very much the same relative position, whereas the posterior end extends rapidly backwards. But this backward growth does not affect the entire area. It seems to be brought about by a continual backward retreat of the surrounding mantle-wall, so that the position of the sulci when once they are laid down is not interfered with.
1 “Kinige Satze iiber die Grosshirnfaltungen,” Centralblatt fiir die Medicin ischen Wissenschaften, No. 36, September 8, 1877. 2 Anatomischer Anzeiger, October 1887, No. 21
The post-central sulcus is much later in making its appearance. Asa rule, it does not show until the middle of the sixth month, or even later. Its development coincides with that of the intraparietal sulcus. It would not be possible for this sulcus to take form at a date much earlier than this, because the ground which it occupies in the insula has hardly been included within the Sylvian area at the time when the two other sulci appear.
The period at which the three radial furrows of the insula can be best studied in their relations to the corresponding sulci on the surface of the cerebral mantle is in the latter part of the seventh month, or the first part of the eighth month. Still it is right to state that exceptional cases are met with, and I have observed foetal brains of thjs stage in which the insula was perfectly smooth. Further, there is good reason to believe that in the development of the sulci and gyri the right insula is usually in advance of the left, and also that the process is greatly retarded in the female brain. Rudinger? has contended that all the convolutions of the cerebrum of the female foetus are backward in their growth as compared with those of the male foetus. Upon this point I have not been able to satisfy myself, because we meet with cerebral hemispheres belonging to the same period of development, and the same sex, which present very different degrees of complexity. In the case of the insula, however, the law certainly does appear to hold good.
The following is a brief account of the condition of the insula in those foetal brains which I have specially examined with the view of determining the development of the gyri and sulci :—
I. 5 to 54 months—6 hemispheres examined.
In three—Insula perfectly smooth (viz., 1 right and 2 left).
In two—Insula with sulcus centralis only—(1 right and 1 left).
In one—Insula with sulcus precentralis and sulcus centralis (right).
II, 5} to 6 months—10 hemispheres examined. In two—Insula perfectly smooth (both left).
1 Ueber die Unterschiede der Grosshirnwindungen nach dem Geschlecht beim Fetus und Neugeborenen Miinchen, 1877.
In fowr—Insula with sulcus centralis only (2 right and 2 left). In four—Insula with sulcus centralis and sulcus precentralis (3 right and 1 left). ITT. 6 to 64 months—8 hemispheres examined. In one—Insula perfectly smooth (right). In one—Insula with sulcus centralis alone (left). In three—Insula with sulcus centralis and sulcus precentralis (2 right and 1 left). In three—Insula with the three sulci (2 right and 1 left). IV. 64 to 7 months—3 hemispheres examined. In each of these the insula showed all the three sulci.
In the latter weeks of intra-uterine life the development of the gyri and sulci on the surface of the insula takes place very rapidly, consequently, at birth, the insula presents very nearly the same convolution pattern that it does in later life. All the details are filled in. Further, the precentral furrow, instead of having fallen back, as Guldberg supposed, to form the sulcus centralis, has in reality moved very slightly forwards, so that it does not lie so accurately in line with the corresponding furrow on the mantle as it did on its first appearance. This is brought about by the formation of that triangular depression which marks off the gyrus brevis secundus from the gyrus centralis anterior (gyrus brevis tertius).
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