III. Obliquely transverse section of the head, passing through the eyeballs

female, set.

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Braune W. An atlas of topographical anatomy after plane sections of frozen bodies. (1877) Trans. by Edward Bellamy. Philadelphia: Lindsay and Blakiston.

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PIROGOFF, in his works which we have frequently cited, has carried out his sections of the human body in three directions, and upon that attempts to establish its configuration. It might be expected, as a matter of course, that such a proceeding could be applicable in geometrical solids, but it is not sufficient in organised bodies, even if one divided such a body into as many thin laminae as possible. In organised bodies oblique sections in individual places show far more than purely sagittal, frontal, and transverse sections.*

This is more particularly the case with the eye. Supposing it be desired to give not merely the relation of the globe, but to include the structures in its immediate neighbourhood, we must aim at bringing into view the connections of the eye with the optic nerve as far as possible up to the brain.

It is of only secondary interest to determine in which horizontal plane behind the orbit the individual portions of the temporal lobe lie. In order to follow the course of the optic nerve as the relations on the skeleton indicate, we must direct the plane of section from the middle of the pupil, obliquely upwards.

The idea of this proceeding had already struck Sommering, and in his monograph (' De Oculorum Hominis de sectione horizontali commentatio,' Getting, 1818) a very useful plate is given, so also in Pirogoff's atlas (fasc. i, tab. iii, 4, 5). As follows from Sommering's work, he employs the

These terms are in frequent use in this work, and imply respectively a vertical section by a plane passing through the structures in an antero-posterior direction, a section produced by a plane also passing vertically, but at right angles to the former, whilst a transverse section implies one made at right angles to the axis of the trunk or extremities horizontal only as opposed to the longitudinal section, without thereby meaning that he adhered to a horizontal plane which was accurately mathematical. I considered it profitable to conduct the section in the same way as Sommering has done, and to carry it up tolerably far back. I convinced myself, however, from a large series of sections, that we are not positively able to determine in what position of the optic nerve or tract the saw will fall from the front inwards. The individual differences of the base of the skull are so numerous that it is impossible to assign to them any definite data.

Only so much is certain, that the optic tract rises up from the chiasma to the corpora quadrigemina considerably more vertically than the optic nerve does to the optic foramen. I was therefore unable to expose the optic nerve thoroughly throughout its entire length, but was obliged to supplement the section by taking off a thin slice of the anterior lobe in order to completely expose the chiasma. Again, a thin layer of fat was removed from the orbit so as to show the entire breadth of the nerve, as the line of section had just missed its upper edge.

It must be further explained that, although the external form of the globe be established, the relations of the lens and iris must be rendered after further sections. The fine dust which even such a thin saw produces was very difficult to remove without causing a change in the relative position of the individual organs of the eye. I therefore froze fresh orbits, sawed through the bones, and then continued the section with a razor. In all cases the eye was thoroughly injected with Thiersch's carmine and glue preparation, in order to give the globe its original expansion. The injection was made from the ophthalmic artery, and in the entirely divided skull which forms PI. Ill, the carotid artery and jugular vein were completely injected with different colours.

It will be noticed from the relations of the brain that the plane of section is obliquely upwards and backwards. In front, owing to the removal of the thin lamina of the anterior lobe, a small portion of the floor of the skull in the region of the crista galli is seen. Behind it are the optic chiasma with a small piece of the optic tract cut obliquely, and further back is a section of the gyrus fornicatus, the superior processes of which lose themselves in the white substance of the cerebrum, and show the small bundles of fibres, the beak-shaped processes which belong to the fornix below.

Externally are the choroid plexuses of the descending cornua. Beneath the white substance of the corpus callosum is a fissure bounded laterally by the optic thalamus and filled up with vessels, and in the middle of it is the pineal body. In this space also is the pia mater passing beneath the corpus callosum to the central portion of the cerebrum. In the middle are the lumina of two large vessels belonging to the great internal veins of the brain, the vense magnse Galeni. These, when followed down with the sound under the splenium to the great veins behind the corpus callosum, and the commencement of the straight sinus, are found to debouch by the two veins here shown. The falx cerebri unites the inferior longitudinal sinus which opens into the straight sinus, with the superior longitudinal which lies further back.

The veins of Galen curve at first upwards, and then bend downwards into the straight sinus. It would be worth while to examine whether during the movements of the brain in respiration any influence is exerted on the venous circulation at this spot, since a change here takes place between the compressible inner veins of the brain and the rigid walls of the sinus.

In front are the optic chiasma and the optic tract cut obliquely, which consequently are more vertical than the plane of the section.

Internally are the grey-edged lentiform nuclei, and behind them lie the sections of the optic thalami. Between them in the middle line is a portion of the third ventricle.

On either side of the chiasma are the internal carotid arteries in section. The ophthalmic arteries are not seen, as they enter the optic foramina beneath the optic nerves. The commencement of the artery of the corpus callosum on the one side and on the other is removed, the lumina seen being those of the artery of the fissure of Sylvius.

The orbits were so divided that the saw passed above the optic foramina, and consequently did not open them. The eyelids were tolerably firmly closed, so that only a small portion of the upper lid came into the section, and the lower remained untouched. The globe was divided almost exactly in the middle. The saw cut the upper border of the entrance of the optic nerve of both eyes. In the orbits it was necessary to remove cautiously a thin layer with the knife in order to expose clearly the optic nerves. These show a slight curve, which on further examination is seen to be associated with a bending downwards, in the form of the letter $, which is their normal position while the eye is at rest. This fact renders it possible that considerable traction on the globe may exist without the optic nerve being stretched in any injurious manner in the various movements of the eyes. A tension of the optic nerves must occur unless they entered the optic foramina in this curvilinear manner. It may be left undecided, and for further investigation, whether by a stretching of the sheath of the optic nerve a lymph motion is brought about in its course. The researches of Schwalbe have established, as I may here point out, that the different tensions of Tenon's capsule .during the action of the muscles of the eyeball may perhaps act in such a way. It is certain that this relative length of the optic nerve is necessary to ensure the position of the globe when the eye is at rest, and if the optic nerve were tense as it passed from the optic foramen to the globe it would be drawn out of this original position by the constant movements of the eyeball.

The optic nerves, which are represented rather too broad in the plate, are about '16 of an inch in diameter. Here they are considerably narrower than when they are within the skull. In the latter situation they are from 2 inch to '24 inch broad. These diameters, however, appear at first sight to be equal on account of the slight length of the nerves. The orbit was not opened to its apex, and is, moreover, proportionally small even for a young person's skull. The length of the nerve from the optic foramen to its entrance into the globe is, according to Henle, about T2 inch, according to Soemmering (' De Oculorum Sect, horiz.,' Gottingae, 1818) l'4inch; in the present plate it is only 1'12 inch.

On the other hand, it shows a complete agreement with the statement of Henle that the centre of the point of entrance of the optic nerve is '16 nearer the middle line from the posterior pole of the axis of the globe. Those portions of the globe which are shown in the preparation require no further explanation. Their symmetrical form appears remarkable, as, according to the statement of Briiche, some considerable want of symmetry exists which is characterised by the fact that the equatorial plane, through the iris, lens, and ora serrata, converges to the nasal side. The globe, together with the cornea, exhibits much more nearly a circle in a section in the horizontal meridian, where the long diameter exceeds the transverse one almost imperceptibly.

It must, however, not be forgotten that preparations such as the present one cannot furnish in this respect any absolute standard. The numerous vessels of the choroid have, as is known, a considerable influence on the form of the globe. As I could not accurately measure the pressure of the injection, and as it was chiefly calculated to throw as much fluid as possible into the vessels of the eye, it is quite possible that it was considerably more powerful than it would be under natural conditions ; great pressure in the vessels of the eye tends to approximate the form of the eyeball to that of a sphere.

It is to be further remarked that freezing does not seem to be the most suitable method of hardening the eye. Influences are thereby exerted which may bring about variations of volume in the watery contents of the globe; however, I know of no other method. It is here not so much a question of determining the forms and position of individual portions of the globe as it is of representing in section the relations of the eye to the orbit and to the other portions of the skull.

The dark fissure in front of the globe represents faithfully the expansion and folding-in of the conjunctival sac. Behind are the attachments of the external and internal recti, which are inserted into the globe beyond its axis of rotation relations which are not represented correctly by Soemmering. It is also evident that the internal rectus is inserted further forwards than the external.

At the inner angle, and lying on the lachrymal bone, is the section of the lachrymal sac, and on the outer angle between the muscle and the bone a small portion of the lachrymal gland.

The relation of Tenon's capsule cannot be represented on account of the small size of the plate ; it was considered that a multiplicity of lines would interfere with clearness. Moreover, the relations of this membrane are not sufficiently made out.

It can be shown that this membrane is in relation with the tarsal membrane, and, with the mass of fat which lies behind it forms a sort of cup, into which the globe is pressed by the influence of the external air. Thus the eyeball moves somewhat like the head of the thigh bone in its socket, the fluid in the lymph spaces playing the part of the synovia. It is not yet shown how the capsule of Tenon is, continued over the sheaths of the muscles where they pass back through it, still less so how the sheaths of cellular tissue stand in connection with it. Such a demonstration would especially be of practical importance as regards the question of the extent of the effusion of blood in the orbit.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Reference

Braune W. An atlas of topographical anatomy after plane sections of frozen bodies. (1877) Trans. by Edward Bellamy. Philadelphia: Lindsay and Blakiston.

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Cite this page: Hill, M.A. (2024, April 18) Embryology Book - An Atlas of Topographical Anatomy 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_An_Atlas_of_Topographical_Anatomy_3

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