Book - Comparative Study of the Sensory Areas of the Human Cortex 3
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Cajal SR. Comparative Study of the Sensory Areas of the Human Cortex (1899)
- 1899 Human Sensory Cortex: 1. The Visual Cortex | 2. Layer of the Large Stellate Cells | 3. The Sensori-Motor Cortex | Figures | Cajal
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Lecture III. The Sensori-Motor Cortex
Comparative Study Of The Sensory Areas Of The Human Cortex.
By Santiago Ramón y Cajal
1899
After the study that we have just made of the visual cortex, we can be more concise in our examination of the motor area. In all cortical regions we notice general structural characters and special features which constitute the physiognomy proper of each cerebral area. Naturally, the latter will be of more interest to us, and they will form the subject of the present lecture.
I shall not stop here to give any history of researches undertaken upon the minute anatomy of the psycho-motor areas. A bibliography of the subject would be very long, tedious, and altogether superfluous, since it has already been provided in the recent studies of Retzius, Hammarberg, and KoUiker. It will suffice to name, among those to whom we are most indebted for a knowledge of the structure of the motor cortex, Meynert, Baillarger, Kolliker, Krause, Betz, Lewis, Golgi, Martinotti, Retzius, Flechsig, Kaes, Hammarberg, Nissl, etc. All these writers have selected the psycho-motor cortex for special study ; and it is safe to assert that all our knowledge of the minute structure of the entire cortex has taken its character from this region, which some writers have denominated " typical." They have done this because it was thought at the time when the fundamental works of Meynert and Golgi appeared that in histological structure the whole cortex corresponded to a uniform design, presenting only unimportant variations in different regions.
Neither have I time to enumerate the layers which have been described for this cerebral region. Their nimiber has varied under the pen of each writer with the animal and the method he has happened to employ. Thus Meynert, who made his observations on man, distinguished five layers ; Stieda, Henle, Boll, and Schwalbe limited their number to four ; while writers like Krause admitted as many as seven. I myself, at the time of my investigations upon the small mammals, recognized four, naming them : (1) molecular layer; (2) layer of small and medium-sized pyramids; (3) layer of large pyramids ; (4) layer of polymorphic cells. This number, deriyed particularly from study of the small mammals, is not valid in the more complicated human cortex. To the four classical layers of smooth-brained mammals we must add one at least, the so-called granular layer of Meynert and other writers. This layer, situated in its very midst, divides the layer of giant pyramids into two, which we may call respectively the external, or superficial, and the internal, or deep, layers of giant pyramids.
To sum up, the following are the layers which it is possible to recognize by Nissl^s method in the human motor cortex (ascending frontal and ascending parietal convolutions). To conform to our scheme in the visual cortex, we have altered the terminology for this region also.
- Plexiform layer of Meynert, molecular layer of some writers).
- Layer of small and medium-sized pyramids.
- External layer of giant pyramids.
- Layer of small stellate cells (granular layer of the authors).
- Internal or deep, layer of giant pyramids.
- Layer of polymorphic cells (fusiform and medium-sized pyramids of certain writers).
Fig. 23. — Saetion of Adult homan motor oortoz« UlMd by Nls«l*f OMtliod (•embcbomAtic). 1. plexiform layer ; 2, layer of small pjrramidi ; 3, layor of mediom•iMd pjrramidi ; 4, oxtorwU layer of ^aat pyramide ; 6, layer of email eUllate oells ; 6, Internal layer of giant pyramids ; 7. layer of polymorphic cells or deep pyramidal layer of medium-eiaad oeUe; 8, layw of fofifonn oaUa.
These layers correspond particularly to the concave portions of the motor convolutions. Over the convexities the gfray matter is thickened especially at the level of the polymorphic layer, which here appears divided into two sub-layers : an external, very rich in pyrramidal and triangular cells (Fig. 23, 7); the other, internal, presenting, besides the heavy bundles of white fibres, fusiform cells disposed in parallel series (Fig. 23, 8).
1. Plexiform Layer
This is similar in structure in the motor and visual areas. It contains, therefore : (1) dendritic arborizations of the pyramidal and polymorphic cells, that is to say, of all the cells of deeper layers (2, 8, 4, 5, 6) except stellate cells of the 4th layer and the cells with short axons scattered through the entire cortex ; (2) terminal arborizations of the ascending axons of Martinotti; (8) the ramifications of the recurrent collaterals which come up from the axons of certain small and mediiun-sized pyramids ; (4) the fibres, terminal and collateral, which arise from the white matter; (5) stellate cells of variable size with short axon which ramifies within the 1st layer ; (6) the special, or horizontal, cells with long tangential dendrites ; (7) finally, neuroglia cells of the two well-known types, with long radiating processes close underneath the pia (Martinotti, Retzius, Andriesen, Be van Lewis, et dl.}, and type with short processes, located at all levels of the plexiform layer (Golgi, Cajal, Retzius, Martinotti).
We shall not enter upon their descriptive details, since all the structures present the same characters here as in the visual cortex. We shall merely add that in the motor cortex the plexiform layer is notably thick. It also contains a greater number of horizontal cells and terminations of the trunks of pyramidal cells (Fig. 25, A^ B^ (7). Its greater thickness arises probably, as Lewis remarks, from the extraordinary number of pyramidal cells in the underlying layers.
2. Layer of Small and Medium-sized Pyramids
(Fig. 24, 2 and 8). — We shall not stop upon these, because they are so well known. Permit me merely to call to mind the fact that their radial trunk, often forked near its origin, makes its arborization in the plexiform layer ; while from the base springs a fine neurite which, in case of the small mammals, we can trace into the white matter. In the child's cortex this is made difficult by the distance, but I have been fortunate on two occasions in following this axon into the medullary substance, where it was continued as a medullated fibre. The neuritic collaterals are also very numerous and a number of them may be seen to recur and make their arborizations in the superficial lamina of the plexiform layer.
Cells with Short Axons
These are numerous, although it does not seem to me that they are so extremely abundant as in the visual region. In Fig. 25 I have reproduced some of these elements which habitually occur in my preparations. We remark especially: a, a large stellate type, whose ascending axon subdivides into horizontal or oblique branches covering a great extent of the layer of small and medium-sized pyramids (Fig. 25, if); 6, a second type of similar form but whose axon forms its terminal arborization very close to the cell (Fig. 25, J?); e, still another form with horizontal axon the superficial branches of which penetrate into the plexiform layer (Fig. 25, 2>); (i, arachniform cells with axons subdivided into dense plexuses (Fig. 25, JP, (?) ; «, fusiform, bipanicled cells, which have been sufficiently described.
Fig. 24. — BBMmbto of laytn of motor eorUz of InCaDt «fed ooo and a half mooUif ; Oolgi't method (leinttcheniaUc). Layert art Dumberad aa follows: 1, plexiform; 3 and 3, small and medium-sized pjramlda; 4, oparfldal fffaat pyramlda ; S. jpiuiiilar or email Btellate eelle ; 6, deep g^Ukl pyramids ; 7, polymoiphle eella, or deep medlnm-alaed pynunids. (la this flcare I have not reprsaented the daepeai portion of the 7th layer^
Having studied all these types and many others in the visual cortex, it is unnecessary here to enter upon a more detailed description. One thing concerning the bipanicled cells I may add, viz., that in the motor cortex there appear to be two kinds: one, small cells provided with slender axon disposed in very delicate vertical pencils (Fig. 25, J7) ; the other consisting of relatively large cells having very long and thick dendrites and with an ascending or descending axon giving rise to terminal arborizations of extreme complexity, producing nests or terminal baskets about the bodies of the small and medium-sized pyramidal cells (Fig. 25, J). Possibly this type, which I take to be a variety of the common bipanicled cell, is present over the whole cortex ; but as yet I have succeeded in finding it only in the motor convolutions of the infant at over one month of age.
Fig. 25. — Cells with abort axons of the plexiform and small and medium-sized pyramidal cell layers from motor cortex of infant aged one month and a few days. A^ B, C, horizontal cells of the plexiform layer ; D, cell with horizontal axon ; E, large cell with very short diffusely subdivided axon ; F, O, spider-shaped cells whose delicate axons form a dense plexus {O) up to the plexiform layer; H, J, bipanided cells.
==3. Soperiidal Layer of Giant Pyramids==Being a continuation by imperceptible gradations of the above, this layer contains the well-known large pyramids of the writers. In addition to the observations of Bets, Lewis, Oolgi, and myself, however, I must add a single detail to their classical description. The radial process varies greatly as to the extent of surface it covers in the plexiform layer. When its dendrites must cover a large surface, the trunk forks near the cell, and the two trunks, deviating at an acute angle, ascend to give rise to two or more terminal sprajrs, in some cases at considerable distances apart. This amounts to saying that certain medium-sized and large pyramids stand related to a large number of nerve fibres in the 1st layer, while other cells of the same size have more limited connections (Fig. 24).
In gyrencephalous mammals, dog and cat, the superficial large pyramids are smaller than in the in&nt. They might be considered as a subordinate element in the layer of medium-sized pyramids. Most frequently the only giant pyramids in the cat occur below the granular layer, — a layer which, I may add, is very slightly developed in this animal, being often blended with the layer of medium-sized pyramids.
The number of superficial, medium-sized, and giant pyramids is very large in the motor area both in imimula and man ; and this is one of its characteristic features. However, the regions designated by Flechsig as association centres possess also a notable number of large pyramids. From this feature alone it would be quite difficult to distinguish the frontal and parietal from the motor convolutions.
The axon of the large and medium-sized pyramids descends, as is well known, to the white matter and is continued as one or two nerve fibres. I must call special attention to the fact that, as shown by my own researches, this fibre may fork usually into a fine branch which goes, probably, to the corpus callosum and a larger branch to the oorpus striatum. This may be easily observed in the brain of a newborn mouse or in one a few days old. It may also be seen that the fibre entering the corona radiata passes beyond the corpus striatum, giving off to it a few collaterals. It is thus well established that the axon of the large pyramids is true projection fibre which takes part in forming the pyramidal tract. But we must be on our guard about accepting the view of certain writers, — v. Monakow, for example, — who ascribe this role, participation in the motor tract, exclusiyely to the giant pyramids, because I have demonstrated beyond all doubt, in the motor region of the mouse and rabbit, that a number of the axons of medium-sized pyramids and many from polymorphic cells also penetrate the corpus striatum. I therefore consider as wholly arbitrary all the opinions which tend to attribute an exclusive function to elements in each distinct cortical layer. In the cortical layers, as well as in the ventral horn of the spinal cord, there occur together elements with axons of very diverse character and connections. The motor cell takes its place beside the associational cell along with the element whose axon or collateral goes to the corpus callosum. There are, accordingly, in the cortex no ^^ sensory layers nor ^ motor layers" ; because, as we shall see in a moment, the great majority of the cells are related, either by their cell bodies or by their radial trunks, to the plexus of sensory fibres. We find thus reproduced the arrangement of the spinal cord, where all the cells, or almost all, come into contact with sensory fibres of the first or second order, and all represent links in the chain of reflex connections.
4. Layer of Small Stellate Cells Granular Latter of the Authari)
Stained by Nissl's method the layer of small stellate cells appears as a great number of nuclei surrounded with little protoplasm which contains a few fine granules of chromatin (Figs. 23, 6, and 24, 5). Most of these elements, the granules proper, are very small and globular or stellate in form. Others, I have observed, are comparable to small pyramids, being of triangular form and having a fine radial trunk. Nor is there any lack of stellate or fusiform cells of considerable size, which call to mind those of the visual cortex. All these elements appear to be mingled. However, in certain places I thought I could discover that the small globular cells are situated chiefly in the external plane of the layer, while the minute pyramids were more numerous in the deeper levels, but there are exceptions to this.
But Nissl's method does not enable us to study the fine processes of these elements. To this end we must have recourse to the ohromate of silrer method, and by its application — especially in case of an infant fifteen to thirty days old, a time at which the reaction is easily obtained — I have been able to demonstrate the extreme complexity of the granular layer. Good preparations show that it consists of elements with very diverse characters, which in spite of their minor differences may be classed into two groups : (1) cells with long axons which extend down to the white matter; (2) cells with short axons which end within the granular layer or in layers above it.
Cells with Long Axons
These may be classed into two varieties, small pyramidal cells and medium-sized stellate cells.
(a) The small pyramid is specially numerous in the deep level of the 4th layer (Fig. 26, A^ B). It has been figured by various writers, notably by Kolliker, although even he does not give us any information on the character of its axon. The cells are ovoid-pyramidal in form. They possess a radial trunk which extends up to the plexiform layer, where it ends in a few very slender varicose twigs without contact granules. It also has a few tiny descending or oblique dendrites which divide repeatedly. Finally, I have very often traced its axon to the white matter, in which it is continued as a slender medullated fibre. From its initial portion arise two, three, or four collaterals, some of which curve upward to distribute themselves through the 4th layer. In some cases the diameter of these collaterals is so large, compared with that of the axon, that they might be considered the real axons.
(6) Stellate Cells. Very hard to stain, and possibly quite scarce. Their dendrites arise from the angles of the cell body and run in all directions, but are distributed exclusively to the 4th layer (Fig. 26, D). Their axons spring from the inferior surface, descend almost in a straight line, and, after giving off a few large collaterals, very frequently arched and recurrent, are lost in the white matter. These interesting cells, exactly similar to the stellate cells of the visual cortex, are also found in the motor cortex of gjrrencephalous mammals, although, to judge from my own preparations, only in small numbers. Their presence would seem to indicate distinctively sensory regions of the brain.
Elements with Short Axons. — These are also very numerous in the infant brain, representing, perhaps, the chief morphological factor of the 4th layer. Several varieties have been distinguished, of which the most common are the following : —
(a) Stellate or Fusiform Cells of Medium Size. Their dendrites diverge in all directions, but chiefly above and below, and end in the midst of the 4th layer. Their axon springs from the superior surface, ascends for a variable distance, and at varying levels of the layer of stellate cells forms an arborization of horizontal or oblique branches of considerable length and distributed exclusively to the 4th' layer. Very often the axon branches in the form of a T before proceeding to its terminal arborization, and from its initial part arise collaterals whose course and tenninatioiis resemble those of the terminal branches. These cells, we may add, correspond in all points to the cells with ascending axons described for the 4th and 5th layers of the visual cortex (Fig. 27, A^ (7, 2>).
(b) Fusiform, Triangular, or Stellate Cells. These are somewhat larger than the preceding. Their axon ascends to the plexiform layer, in which it subdivides and terminates. In its ascent it supplies a few collaterals to the 4th and 3d layers. These elements, as we see, correspond to the so-called cells of Martinotti. In a few cells of this class the axon possibly does not reach the first layer, becoming lost in the layers below (Fig. 27, A).
(c) Small Stellate or Spider-shaped Cells. These possess fine and richly subdivided dendrites and also a neurite, which forms a very rich arborization close to the cell (Fig. 27, ^).
(d) Bipanicled Cells. These have the characteristics already described in our study of the visual cortex.
(e) Finally, in the cat and dog I have found a few stellate cells with very numerous dendrites, whose descending neurite forms a very dense and complicated arborization, for the most part in the 4th layer, but in some cases extending down to the deep layer of giant pyramids. Possibly these cells are homologous to the spider-shaped cells in man, which they resemble in the extraordinary richness of the plexus formed by the axon. It would then be necessary to suppose, however, that in the cat and dog these cells are much larger than in man.
Fig. 26.— Cells with long axons from 4th layer of motor cortex of infant aged one month. A, Bt C, small pyramidal cells; D, large stellate cell; JS, medinm-siaed pyramid; a, axon; b, e, large descending collaterals.
Fig. 27.— CeUi with abort azoM Iron 4th lajv of motor ooffUzollBlkiii. J. B, Cotlk^ttolImto or fntllorm, with fttoendliif axon dlTided Into long borixontiU hranebes ; E, arttchnilorm ooU ; F, eell with ajan distribotod to layer of modiam-ilsed pyramkU.
In order to complete my description, permit me to add that there is no lack of ordinary pyramidal cells, in some cases large, scattered irregularly in the 4th layer (Fig. 26, E). In mammals like the cat and dog, and to a much greater degree in the rabbit, the profusion of pyramidal cells obscures our picture of the granular layer.
Sensory Nerve Plexus of the 4th Layer. — One of the most significant facts which I have discovered in the motor cortex is a plexus of very large fibres whose numerous subdivisions occupy the 4th layer and extend even into the 2d and 3d. They probably enter the cortex from the corona radiata. As early as in my first work I called attention to these fibres as being different in diameter, direction, and origin from axons of pyramidal cells, but at that time I had not succeeded in determining the region to which they are peculiar or the precise place of their termination in the cortex. My recent researches upon the brain of man and also small mammals enable me to add a few details to my description of some years ago (Fig. 28).
First of all, I have been able to determine exactly their origin and position in the brain. These are both easy to observe in the brain of a rabbit at birth and still better in that of a mouse a few days old. In the mouse it may be seen especially well that certain large fibres (called by Kolliker, who has confirmed their existence, fibres of Cajal) proceed from the corpus striatum, enter the white matter, and often extend horizontally in it for great distances. In their course they throw off long collaterals, which penetrate into the overlying gray matter. All these collaterals, as well as finally the orig^al axon itself, ascend through the polymorphio layer, dividing once or twice, then, passing obliquely through intervening layers, form an arborization of heavy fibres within the layers of small, medium-sized, and large pyramidal cells. However, in the rat and rabbit these branches are most numerous in a relatively superficial plane, which corresponds probably with the granular layer of the human brain, — a layer that is not differentiated in the small mammals. We also find a relatively small number of branches that ascend to the plexiform layer. As to the cortical distribution of this plexus, we may also place on record a fact of interest. It never covers the whole cortex. It begins to appear some distance from the median fissure and disappears below long before reaching the olfactory area or limbic lobe. I have never observed it in the cortex of this sulcus, nor in the anterior portion of the frontal lobe, nor even in the region of the auditory or visual centres.
Fig. 28. — Pkzm of baAvy mnaorf fibres from motor oortcz of cmi S days old. A, plsxlfora layer ; B, Uysr of small and medlom-siiwl pyramids ; C and />, layers of i^ranales and soperftclal layer of ffiant pyramids; E, deep layer of f(iant pyraniidi; F, layer of pi>lymorphlc cells . a. fibre from white matter; 6, ascending collateral ; c, rarlcnse terminal arbi»riiAtion ; <f. fibre directed to tbe plexiform layer, which appears to be distinct from the lar^se Ahrcn.
I shall return to this matter in a future investigation, for I think it merits most thorough study ; because, if it can be confirmed in a positive manner and by other methods, we shall possess a criterion by which to distinguish between areas of association and projection in the cortex. The projection areas will probably be found to be not, as Flechsig thinks, those possessing fibres that go to the corpus striatum (since Dejerine and others have discovered these fibres in the so-called association centres) but those receiving sensory fibres. At the same time, the association centres will be characterized by the absence of these direct sensory connections. At any rate, I believe that even in the brain of the smallest mammal there are areas, of small extent it may be, specialized to store up the images or residues of the sensory projection centres. It would be most astounding if the brains of the small mammals possessed a different architecture from that of man, taking into consideration the fact that all the senses have the same essential structure in all mammals and that memory — visual, tactile, muscular, etc. — is just as necessary to their lives as to our own.
The sensory plexus is highly developed in gyrencephalous mammals and in man. I have found it well impregnated in the brains of infants at birth and a few days old. Here it appears made up of large fibres having an oblique direction and a flexuous or even staircased course. After dividing several times in the 6th and 5th layers they give rise to a singularly extended arborization of horizontal fibres distributed chiefly to the layer of granules or small stellate cells. We thus see in the motor cortex, M was the case in the yisoal, that the layer of granules is the principal focus of sensory impressions* From this terminus they are propagated by the numberless cells with short ascending axons to the layers above and especially to the medium-sized and giant pyramids. However, it must be acknowledged that the sensory plexus is not so narrow and well defined as the optic. For, although its greatest density occurs in the 4th layer, its terminal branches divide in their ascent to the superficial layer of medium-sized and giant pyramids. The fibres which extend up to the small pyramids in man are not numerous. It is for this reason that I cannot agree with Bevan Lewis in ascribing to them sensory functions. I do not wish to be understood to deny the sensory function of the small and medium-sized pryamids. According to my view, all the cells of the motor cortex are sensory because they all, possibly, come into contact either directly (cells of the 8d, 4th, and 5th layers) or indirectly, through the intervention of cells with short axons, with sensory terminal arborizations. But, since some cells send their axons to the pyramidal tracts, we are able to distinguish them as $en$ari'^notor eetU of the far$t order. The others, which send their neurites to other motor areas of the brain, possibly effect contact with sensori-motor cells of the first order located elsewhere. These cells of indirect sensori-motor communication we may be warranted in calling eeneori'inotor eeU$ of the eecond order. It goes without saying that this distinction is purely hypothetical ; for no method enables us to determine the precise point within the brain where the axons of the pyramidal tracts of the corpus callosum or of bands of association fibres form their terminal arborizations.
6. Layer of the Giant and Medium-slzed Pyramids
In the adult human brain stained by Nissl's method, a section of the motor cortex reveals, below the granular layer, a layer of plexif orm or granular aspect filled very thickly, but in no particular order, with a few giant and a great number of medium-sized pyramids (Fig. 29).
Usually the giant pyramids are located near the 4th layer, forming there a few irregular ranks. Impregnated by Oolgi's method, they appear similar to the same cells in other regions of the cortex, but differ in a few particulars. The body is generally conical, very much elongated, giving rise at the apex to a large trunk, often dividing near the cell, which terminates in the 1st layer in the usual manner. A group of long complicated dendrites diverges from its base, and from the sides spring several very long horizontal processes which subdivide into terminal brushes, and these, intertwining with similar structures from neighboring cells of the same level, form a dense and very characteristic protoplasmic plexus. It is the same arrangement we already know so well in the visual cortex, except that, instead of one plexus, there are many. The axon is large and, after giving oflf very long collaterals to the 5th and 6th layers, it passes on to become a medullated fibre of the white matter.
Fig. 29. —Deep layer of giant pyramidal cells from motor cortex of infant aged 20 days. At Bt pyramidal cells ; D, C, elements with short axons.
The medium-sized pyramids are very numerous, much scattered, and occur in greatest profusion in the lower levels of the layer. They do not differ in character from the giant pyramids, except as to the lateral somatic dendrites, which are few and not characteristic.
Besides the pyramidal cells the 5th layer contains a few other kinds of elements. From the point of view of their morphology the following are the more striking types.
(a) Cells which form Terminal Ne$U. — These cells, very similar to those which give rise to the basket fibres of the cerebellum, are most numerous in the 5th layer between or below the giant pyramids. I have found them also in the layer of granules or small stellate cells.
Their volume is small, similar to that of a small pjrramid, and in form they appear stellate or triangular with very long and much-branched varicose dendrites. The neurite, however, presents the most distinctive feature. It ascends, forking close to its origin, and breaks up into a ramification of very many branches, ascending, oblique, or horizontal. After a few subdivisions, all these branches make their way to the giant and medium-sized pyramids to form very complicated varicose arborizations close around their cell bodies and principal processes, after the manner of the terminal baskets of the cerebellum or the nests found in Deiter*s nucleus. Elach nest contains arborizations from several cells, and each basket cell helps to form a large number of nests (Fig. 30, <f).
(b) Cells with a Difftuely Branched Ascending Axon. — This is a fusiform or stellate cell located at different levels of the 5th layer, to which it sends its dendrites. The axon ascends to the superior limits of the layer where it forks, and its terminal branches form a loose horizontal arborization of an enormous extent and connected probably with the deep giant pyramids (Fig. 29, (7, 2>).
(c) Small Pyramid with Arched Axon: — This cell, which I have studied particularly in the motor cortex of the cat, is entirely similar to the element which we found in the 6th and 7th layers of the visual cortex. The cells possess a fine dendrite which ascends to the first layer, where it ends in a very modest and delicate arborization. Their axon descends and, after giving off a few relatively long recurrent collaterals, appears to fork and end in the midst of the 5th layer. The branches which spring from the bend of the arch descend in some cases, but I have not been able to trace them down to the white matter.
(d) Cells with Long Ascending Axon. — These are fusiform or triangular cells with long polar dendrites which never reach the first layer.
Their axon arises from the superior surface of the cell, and, after giving off a few branches to the 5th and 4th layers, it continues its ascent to the plexiform layer and there makes its terminal arborization.
6. Layer of Polymorphic Cells
This layer contains the same elements as the layer of the same name (9th) in the visual cortex (Fig. 81), that is to say, fusiform cells with two long polar dendrites, triangular cells, and true pyramids. Their axons all go to the white matter. Their ascending trunks, which are never lacking, become very attenuated on account of the branches given off while passing through the 4th layer and reach the 1st layer as an exceedingly delicate fibril, which ends in a fine, slightly extended, notably varicose dendritic spray.
Fig. 30. Perioellalar tenninal arborisationB from the deep layer of giant pyramids, motor oortez (ascending frontal eonyolution) of infant aged 25 days, a, axons giving rise to oblique and horizontal branches; &, c, d, terminal nests.
In Fig. 81, 1 have reproduced the principal types of cells found in the polymorphic layer. Besides the medium-sized pyramidal and triangular types having long descending axons (Fig. 81, A^ J?), there occur other forms in great numbers. These are fusiform or triangular cells whose axons penetrate into the superposed layers^ furnishing to them a great number of branches. Some of these axons seem to end in the deep layer of giant pyramids, but others appear to pass beyond this. Finally, there is no lack of arachniform cells (Fig. 81, J), cells with short axon of the sensory type of Golgi, whose axons form terminal arborizations in the layer under consideration. I may add that I have found in two cases giant stellate cells with heavy horizontal axon which gives off collaterals (Fig. 31, JST). I do not know the ultimate fate of this process and am unable to say whether these scattering cells form a constant feature of the motor cortex.
Fig. 31. — PrliidpaltyiiM of poljiDOfplite otUs from BoloreorUs of Infant atid so days. A, B^ Mtto with lonff axons aztMUaf to whito MUtar ; C, i>, JT, fnalform eaUa with aapandinf axon ; H, flaat staUata oall.
Cortex of Acoustic, Olfactory, and Associational Areas
Unfortunately, my own researches are not as yet in a very advanced state in regard to these important cortical centres. So that any information that I can give must necessarily be fragmentary and of little value.
The acoustic resembles exactly the motor cortex as to general arrangement of cells and layers, but differs from it in a few peculiarities : (1) by the fineness of the fibres forming the plexus at the level of the layer of granules or small stellate cells ; (2) in the profusion of bipanicled cells with their very delicate and complicated neuritic brushes ; (3) above all, by the presence of certain special cells scattered irregularly through the entire thickness of the cortex. The very large axon of these special cells extends in a horizontal or oblique direction, but I have not yet been able to determine exactly its manner of termination. These large cells are fusiform and lie horizontally. From their polar dendrites spring a number of fine ascending branches, which subdivide repeatedly but do not extend up as far as the plexif orm layer.
The olfactory cortex, that of the limbic lobe, is characterized by the following peculiarities : (1) the enormous development of the plexiform layer and the presence in it, in addition to its usual structures, of the antero-posterior fibres that come from the external root of the olfactory tract; (2) the absence of the layers of small pyramids and granules; (3) the presence of certain large horizontal cells below the plexiform layer ; (4) the peculiar form of the medium- and large-sized pyramids which emit from the deep end of the cell body a brush consisting of numerous much subdivided dendrites; (5) above all, the fact that the sensory plexus, i.e. the fibres which come from the olfactory bulb, makes its terminal arborization exclusively in the plexiform layer and in the most superficial portion of that layer, corresponding to that of the small pyramids. This significant fact, brought to light by the studies of Calleja^ shows us that the sensory fibres do not end in the same level of the cortex in all regions. Hence, the layer specialized to serve as substratum for the phenomena of sensation may change its position in different sensory areas.
Our task is now drawing to its dose. My work upon the topographical structure of the cortex has been fragmentary and leaves much to be desired. Many things, in fact, are still undiscovered. But, despite the very incomplete state of my researches and the narrow limits of the field they cover, I may draw a few anatomico-physiological conclusions, of which the chief are the following : —
And first, as to the hierarchy of centres in the cortex of the human brain, comparing it with the mammalian brain, we may call to mind that, while it does not contain wholly new elements, it presents very distinctive characteristics, to wit : —
- The enormous development of the horizontal cells of the plexiform layer and the considerable length of their so-called tangential fibres.
- The great abundance of cells with short axons scattered throughout the whole cortex, oells which form special varieties by reason of differences in their forms and the directions of their axons.
- The presence of cells with short axons, very slender (bipanicled spider oells), with terminal arborizations whose delicacy ia not approached by anything found in any animal.
- The considerable development of basilar dendrites of the pyramidal oells.
- The presence among the mid-layers of the cortex of a formation of so-called granular cells, a kind of focus occupied by enormous numbers of pyramids with short axons descending, arched, and ascending. This granular formation is present in gyrencephalous mammals, but in them it is very poor in cells with short axons and in small pyramids. In the smooth-brained animals it is almost wholly lacking.
The human cortex has evolved, accordingly, along three different lines: by multiplying cells with long axons and, above aU, those with short axons ; by decreasing the volume of cells and the diameter of certain fibres in order to make possible within the limits of space a delicate and greatly improved organization ; finally, by varying and infinitely complicating the external morphology of the nerve elements, undoubtedly with the purpose of multiplying, in correspondence with their complexity, functional associations of all kinds.
As to differences and analogies in regional structure, the following propositions may be regarded as established: —
- The sensory as well as the so-called associational areas are made up by a combination of two orders of structural factors. The first order consists of common factors, which show very little modification. They are represented by the plexiform layers and the layers of pyramidal and polymorphic cells. The second order comprises special factors, structures peculiar to each cortical area. Their chief anatomical feature resides especially in the granular layer and is related mainly to the presence of particular centripetal fibres and of special types of cells with long axons (stellate cells of different kinds).
- It seems probable that the common factors perform functions of a general order concerned, possibly, with ideas of representations of all kinds of movements related to the special sensations of which the cortical region is the seat. It seems also probable that the special anatomical factors of the sensory areas perform the function of elaborating specific sensations (sensation of seeing, hearing, etc.) and also of conveying sensory residues to the so-called association centres, where they may be transformed into latent images.
- Each sensory cortical centre receives a special category of nerve fibres (fibres of centried sensory tracts). Their cells of origin, as has b6en shown by the researches of v. Monakow, Flechsig, v. Bechterew, and many others, reside in the particular nuclei of the medulla, corpora quadrigemina, and optic thalami. It is precisely the presence of these sensory fibres of the second order that constitutes the prime anatomical characteristic of the centres of sensation or projection.
- The absence of these sensory fibres, which come from the corona radiata, may be. used in aU mammals to distinguish the so-called association centres. These centres, which exist even in the mouse, also have a nerve fibre plexus distributed among their median layers (layer of granules in the association areas in man). The fibres, however, which constitute them are very fine and appear to come from sensory centres of the brain. Possibly the cells about which these sensorio-ideational fibres terminate represent the substratum or, at any rate, the first link in the chain of nerve elements whose function is the representation of ideas.
- Since we have seen that each afferent fibre in the sensory cortex comes into contact with an extraordinary number of nerve cells apparently scattered without any order, we must suspect that these relations conform to the preconceived design of a well-determined and constant organization.
As, at present, it seems to be impossible to discover these relations, we may surmise that each sensory fibre comes into contact, directly or through other cells, solely with those pyramids whose stimulation is necessary in order to effect, after the manner of the reflex arc, movements coordinated and intentional. We may also surmise (supposing* that the stellate cells of the tactile and visual cortex form the link between the sensory and ideational centres) that each sensory afferent fibre, bringing a unit of sensation (the impression received by a cone of the retina or by the terminal arborization of any peripheral nerve fibre), enters into relation exclusively with the group of nerve cells entrusted with the function of conveying this impression to a particular point in the associational cortex.
Many other hypotheses are possible, but I must conclude for fear of tiring your kind and sympathetic attention and exhausting your patience. I fear that I have already made too free use of hypotheses and have pretended to fill the gaps of possible observations with arbitrary suppositions.
It is a rule of wisdom, and of nice scientific prudence as well, not to theorize before completing the observation of facts. But who is so master of himself as to be able to wait calmly in the midst of darkness until the break of dawn ? Who can tarry prudently until the epoch of the perfection of truth (unhappily as yet very far off) shall come ? Such impatience may find its justification in the shortness of human life and also in the supreme necessity of dominating, as soon as possible, the phenomena of the external and internal worlds. But reality is infinite and our intelligence finite. Nature and especially the phenomena of life show us everywhere complications, which we pretend to remove by the false mirage of our simple formula, heedless of the fact that the simplicity is not in nature but in ourselves.
It is this limitation of our faculties that impels us continually to forge simple hypotheses made to fit, by mutilating it, the infinite universe into the narrow mould of the human skull — and this, despite the warnings of experience, which daily calls to our minds the weakness, the childishness, and the extreme mutability of our theories. But this is a matter of fate, unavoidable because the brain is only a savings-bank machine for picking and choosing among external realities. It cannot preserve impressions of the external world except by continually simplifying them, by interrupting their serial and continuous flow, and by ignoring all those whose intensities are too great or too small.
I cannot conclude this, my third and last lecture, without a word of tribute to this great people of North America, — the home of freedom and tolerance, — this daring race whose positive and practical intelligence, entirely freed from the heavy burdens of tradition and the prejudices of the schools, which weigh still so heavily on the minds of Europe, seems to be wonderfully endowed to triumph in the arena of scientific research, as it has many times triumphed in the great struggles of industrial and commercial competition.
- 1899 Human Sensory Cortex: 1. The Visual Cortex | 2. Layer of the Large Stellate Cells | 3. The Sensori-Motor Cortex | Figures | Cajal
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Cite this page: Hill, M.A. (2024, April 25) Embryology Book - Comparative Study of the Sensory Areas of the Human Cortex 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Comparative_Study_of_the_Sensory_Areas_of_the_Human_Cortex_3
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