صور الصفحة
PDF
النشر الإلكتروني

We come now to the brain. Anatomists have erred greatly in taking the human brain as the type or standard of all others, and judging of the presence or absence of its various parts in other animals by contrasting their appearances with those of man. The only safe rule is to begin at the simplest brain which we can find, and to trace the various modifications and additions which it receives as we ascend in the scale of creation, and to be guided in this investigation not by mere form or situation, but by the function which it performs. There is no other way of escaping the serious error of mistaking one part for another totally different. Thus, if we were not guided by the function which accompanies it, it would be utterly impossible for any anatomist to perceive that the olfactory nerves of the skate, flounder, fowl, seal, and ox, were in reality analogous masses. And it is quite certain that the different parts of the brain are, to say the least, not less modified in different animals than the olfactory nerves alluded to. Hence it is in vain to expect a perfect comparative ana. tomy of the brain, without a knowledge of the affective and intellectual faculties of animals, and of the functions perform ed by the different cerebral organs.

The brain is an aggregate of many parts, each performing a distinct function. But, speaking generally, these may be divided into two masses, one constituting the organs of the intellectual and the other those of the affective faculties. The anterior pyramidal bodies are the rudiments of the for mer, and the olivary and part of the restiform bodies are the rudiments of the latter ; the separation between them being pretty distinct from the medulla up as far as the optic thalami and striated bodies.

The pyramidal bodies are scarcely perceptible in birds, and still less in fishes and reptiles, but they are apparent in the lowest of the mammalia. In man they usually commence about twelve or fifteen lines below the annular protuberance, and increase in size as they ascend. The primary fibres of each do not issue from the same side as that on which they lie, but uniting, in the first instance, into two, three, or” as many as five little cords, they cross the mesial line of the body one above another from below upwards; those of the right side arising from the left, and vice versa. This is termed their decussation. This structure was accurately described in 1709 by Mistichelli, and afterwards by Petit, Lieutaud, Santorini, and Winslow. Among the moderns, Sabatier, Boyer, Dumas, Bicbat, and Chaussier, deny its existence. To see it, make a slight cut through the pia mater in the mesial line without implicating the cords beneath, then separate the edges gently, and the decussation

will appear.

A few fibres are detached from each pyramidal across to the olivary body of the same side, as if to place the two in communication.

The pyramidal bodies proceed upwards, and, just before entering the pons varolii, are somewhat contracted in their thickness. Immediately on entering it, they separate into several bundles, and are mingled with cineritious substance. Many new fibres arise and join the others, all advance, some in layers, and some intersecting the bundles of the annular protuberance. The pyramidal bodies are thus so much increased, that, on emerging from the pons, they compose the

, anterior and outer two-third parts of the cerebral crura.

To see the passage of the pyramidal bodies, an incision of about a line in depth must be made across the transverse fibres of the pons. These fibres, are now to be separated from the longitudinal layers by pushing the former aside by the handle of the scalpel, cautiously placed under them; the longitudinal fibres will then be distinctly seen. The great bundles called crura of the brain thus

to be, in part at least, a continuation of the pyramidal bodies increased in size and perfection. These crura as they advance also contain cineritious matter in their interior, from which other and additional fibres arise, and go to increase their size.

appear

In the mammalia, the crura are evidently divided into two parts, viz. an anterior and external, and a posterior internal mass, separated by a superficial furrow. In man the first part belonging to the pyramidal bodies is much larger in proportion to the others than in the inferior animals. But before pursuing farther the pyramidal portion of the crura, let us examine the posterior portion.

The medulla oblongata is composed, besides the pyramidal, of the olivary and restiform bodies. The restiform bodies contain the origins of the primary portion of the cerem bellum and of the vocal, glossopharyngeal, facial, and trigeminal nerves. The remaining fibres of these, and the fasciculi of the olivary bodies, mount behind the ganglion of the pyramidal bodies in the annular protuberance, and joining themselves with, aid in the completion of the cerebral crura. In this course, they gain some increase in size, inconsiderable, however, compared with that of the pyramidal bundles. The olivary bodies are themselves true ganglions, and present the general forms or modifications observable in the ganglion or dentated body of the cerebellum, and their size varies greatly in different individuals. They contain both cineritious and fibrous substance. ! These anterior and posterior portions of the crura cerebri are the roots or primary bundles of the hemispheres; but they are of course afterwards immensely increased in size. The first great augmentation takes place at the upper extremity of the anterior portion, where the optic nerve winds over it, or, in other words, at the striated bodies. After this, the fibres advance of unequal lengths, and, expanding into layers covered on their peripheral extremities with cineritious substance, ultimately form the inferior, anterior, and external convolutions of the front and middle cerebral lobes.

To show that the lower and inner convolutions of the middle lobe are formed by the anterior and outer bundles of the crura, the middle lobe must be removed, which, from the depth of the fissure of sylvius, is easily effected. Some convolutions formerly concealed will be brought into view, and the bundles mentioned will be seen to belong to the middle lobe and anterior part of the posterior lobe.

If the entire outer part of the striated bodies be removed, the manner in which the convolutions, situated along the middle region of the hemispheres on a level with the temples, arise from the bundles in continuation with the pyramidal bodies, will be made apparent. The pyramidal bodies, their ganglions in the annular protuberance, the anterior and outer portions of the cerebral crura, and the convolutions in whichi their bundles terminate, are always developed in the direct ratio of each other.

The posterior and inner bundles of the crura plunge into a thick, massy, and firm ganglion, flattened in the middle, and unequal above, known by the erroneous appellation of optic thalamus. These ganglions are developed in the direct ratio of the convolutions depending on them, and forming the upper and posterior parts of the hemispheres. In the inte rior of these ganglions there are a great number of very fine nervous filaments; these unite at its superior. edge into bundles, which then diverge towards the convolutions in the

form of rays.

The two portions of the crura cerebri may be separated from each other, either by a blow-pipe or a stream of water before they enter their ganglions ; but after that any farther partition becomes impossible. The anterior bundles of the thalamus traverse the striated bodies, and are distinctly seen. The masses styled optic thalami and striated bodies, therefore, are true ganglions, in which the primary bundles of the brain are increased in their progress to completion in the convolutions. The faultiness of the ordinary mode of dissection will now be apparent, which, instead of tracing the masses from their rudimentary state upwards to completion, mutilates and destroys the parts in their complete state.

One circumstance it is important to remark. The nervous fasciculi are LESS NUMEROUS BUT LARGER in the posterior and

middle than in the anterior region. In the latter they ARE VERY NUMEROUS, BUT ALSO VERY SMALL. This explains why the organs situated in the forehead are observed to be smaller and more numerous than those which lie in the occi. pital region.

Drs Gall and Spurzheim were the first to discover the true purpose of the optic thalami, and to ascertain the real origin of the optic nerve from the quadrigeminal bodies.

The convolutions of the brain consist internally of white fibres, covered externally with cineritious substance. These white fibres are of unequal length. Many of them terminate almost immediately beyond the walls of the ventricles; others extend to distances greater as they run more towards the centre of the convolutions; and it is in consequence of this structure that convolutions appear, the short fibres reaching only to the bottom of the anfractuosity, and the long ones to the surface of the convolutions.

When a convolution is cut across vertically, the white substance will be observed of greater thickness at the bottom than at the top. This is explained by the different lengths and terminations of the fibres already mentioned. A clean cut shows the white substance as a simple mass; but it may nevertheless be shown to consist of two distinct layers meeting in the middle line, and simply agglutinated by a very delicate nevrilema ; and this structure is of great importance in understanding the pathological appearances of the brain, as in hydrocephalus, in which the convolutions are unfolded without destroying their functions.

It was, in fact, a case of this kind that led Drs G. and S. to investigate the structure of the brain long after their discovery of many of the organs. A woman of fifty-four; whose

A head was greatly enlarged, was still as active and intelligent as her companions; whence Dr Gall inferred, as Tulpius had done before him, that the brain could not be disorganized or destroyed in the way that was generally supposed, but that its structure must be very different from what it was believed

Vol. IV. -No XIII.

« السابقةمتابعة »