Histologists are generally of opinion, that the central portion of each nerve-fibre differs from the peripheral: the former has been termed by Rosenthal and Purkinje the axis-cylinder; the latter is the medullary or white substance of Schwann, and to it the white colour of the cerebro-spinal nerves is chiefly due.

The researches of histologists have shown that vesicles or cells

containing nuclei and nucleoli, and called also nerve corpuscles and globules and ganglion corpuscles and globules, are the essential elements of gray or vesicular matter. These are found in the nervous centres, mingled with nerve-fibres, and imbedded in a dimly shaded or granular substance. They give to the ganglia and to certain parts of the brain and spinal cord the peculiar grayish or reddish-gray appearance by which they are characterized. They are large nucleated cells, filled with a finely granular material; some of which is often dark, like pigment;-the nucleus, which is vesicular, containing a nucleolus. The marginal figure (Fig. 28) represents some that have a regular outline. Others, as in Fig. 29, are caudate or stellate, and have tubular processes issuing from them, filled with the same kind of granular matter as is contained in the corpuscle.

Stellate or Caudate Nerve Corpuscles. After Hannover.

a, a. From the deeper part of the gray matter of the conrected towards the surface of the organ. b. Another from

volutions of the cerebellum. The larger processes are dithe cerebellum. c, d. Others from the post-horn of gray matter of the dorsal region of the cord. These contain pigment, which surrounds the nucleus in c. In all the specimens the processes are more or less broken. Magnified 200 diameters.

The gray substance is not always at the exterior, nor the medullary in the interior. In the medulla spinalis, their situation is the reverse of what it is in the brain. In the invertebrata, the gray matter forms the nuclei of the ganglia, which are the centres of the nervous system; and the true spinal System, which occupies the interior of the spinal cord, has been regarded as a chain of similar ganglia. It is the organ, as already shown, of the excito-motory nervous function. Ruysch consider

ed, that the gray portion owes its colour to the blood-vessels that enter

2

it; and, in this opinion, Haller, Adelon, and others,3 concur; but this is not probable, and it has not been by any means demonstrated. The medullary portion has the appearance of being fibrous; and it has been so regarded by Leeuenhoek, Vieussens, Steno, and by Gall and Spurzheim. Malpighi' believed the gray cortical substance to be an assemblage of small follicles, intended to secrete the nervous fluid; and the white medullary substance to be composed of the excretory vessels of these follicles; and an analogous view is entertained by most physiologists of the present day,-the gray matter at least being regarded as the generator of the nervous influence; the white matter as chiefly concerned in its conduction. Gall and Spurzheim conjecture, that the use of the gray matter is to be the source or nourisher of the white fibres. The facts, on which they support their view, are, that the nerves appear to be enlarged when they pass through a mass of gray matter, and that masses of this substance are deposited in all parts of the spinal cord where it sends out nerves; but, Tiedemanns has remarked, that in the foetus the medullary is developed before the cortical portion, and he conceives the use of the latter to be-to convey arterial blood, which may be needed by the medullary portion for the due execution of its functions. After all, however, it must be admitted with Dr. Allen Thomson,' that the general conclusion deducible from all the facts would seem to be, that whilst the gray fibres predominate in the organic or sympathetic nerves, and the tubular fibres in the cerebro-spinal nerves, these two elements are mixed, in various proportions, in the great divisions of the nervous system; and that, therefore, these divisions, although, in a great measure, structurally different, are not altogether distinct from, or independent of, each other. "But"-he properly adds "in regard to the whole subject of the structure and nature of the different varieties of the nervous texture, it is unquestionable that much still remains to be ascertained by laborious investigation."

Sir Charles Bell10 affirms, that he has found, at different times, all the internal parts of the brain diseased, without loss of sense; but he has never seen disease general on the surface of the hemispheres without derangement or oppression of mind during the patient's life; and hence he concludes, that the vesicular matter of the brain is the seat of the intellect, and the tubular of the subservient parts." A similar use has been ascribed to the vesicular portion, from pathological observations, by MM. Foville and Pinel Grandchamp.12 This view would afford considerable support to the opinions of Gall, Spurzheim, and others, who consider the organs of the cerebral faculties to be constituted of ex

1

1 Oper. Amstel., 1727.

2

Physiologie de l'Homme, 2de édit., i. 208, Paris, 1829.

3 Carpenter, Human Physiology, p. 81, Lond., 1842.

4 Todd, Cyclop. of Anat. and Physiol., Pt. xxv. p. 647, Lond., 1844.

5 Philos. Transact., 1677, p. 899.

• Recherches sur le Système Nerveux en général, et sur celui du Cerveau en particulier, avec figures, Paris, 1809.

7 Oper. Malpighii, and Mangeti Bibl. Anat., i. 321.

8 Anatomie und Bildungsgeschichte des Gehirns, mit Tafeln, Nürnberg, 1816.

9 Outlines of Physiology, Pt. i. p. 155, Edinb., 1848.

10 Anatomy and Physiology, 5th American edit., by J. D. Godman, p. 29, New York, 1827.

"See two interesting pathological cases, confirming this view of the function of the gray matter, by Dr. Cowan, in Provincial Medical and Surgical Journal, April 16, 1845.

"Sur le Système Nerveux, Paris, 1820.

pansions of the columns of the spinal marrow and medulla oblongata, and to terminate by radiating fibres on the periphery of the brain; as well as to those of M. Desmoulins,' and others who regard the convolutions as the seat of the mind. We have, however, cases on record, that signally conflict with this view of the subject; in which the cortical substance has been destroyed, and yet the moral and intellectual manifestations have been little, if at all, injured. Many years ago, the author dissected the brain of an individual of rank in the British army of India, in the anterior lobes of which neither medullary nor cortical portion could be distinguished,—both one and the other appearing to be broken down into a semi-purulent, amorphous substance; yet the intellectual faculties had been nearly unimpaired; although the morbid process must have been of some duration.

The encephalon affords us many striking instances of the different Fig. 30.

Circle of Willis.

1. Vertebral arteries. 2. Two anterior spinal branches uniting to form a single vessel. 3. One of the posterior bellar. 6. Basilar artery giving off its transverse branches to either side. 7. Superior cerebellar artery. 8. Posterior cerebral. 9. Posterior communicating branch of the internal carotid. 10. Internal carotid, showing the curvatures it makes within the skull. 11. Ophthalmic artery divided across. 12. Middle cerebral artery. 13. Anterior

spinal arteries. 4. Posterior meningeal. 5. Inferior cere

cerebral arteries connected by, 14. Anterior communicating artery.

effects produced by sudden, and by gradual interference with its functions. Whilst a depressed portion of bone or an extravasation of blood may suddenly give rise to the abolition of the intellectual and moral faculties, gradual compression by a tumour may scarcely interfere with any of its manifestations.

The circulation of blood in the encephalon requires notice. The arteries are four in number, two internal carotids, and two vertebrals: to these may be added the spinal or middle artery of the dura mater, arteria meningæa media. The carotid arteries enter the head through the carotid canals, which open on each side of the sella turcica, or of the chiasma of the optic nerves. The vertebral arteries enter the head through the foramen magnum of the occipital bone; unite on the medulla oblongata to form the basilary artery, which passes forward along the middle of the pons varolii; and, at the anterior part of the pons, gives off lateral branches, which

1 Anatomie des Systèmes Nerveux des Animaux à Vertèbres, p. 599, Paris, 1825.

inosculate with corresponding branches of the carotids, and form a kind of circle at the base of the brain, which has been called circulus arteriosus of Willis. The passage of the blood-vessels is extremely tortuous, so that the blood does not enter the brain with great impetus; and they become capillary before they penetrate the organ, an arrangement of importance, when we regard the large amount of blood sent to it. This has been estimated as high as one-eighth of the whole fluid transmitted from the heart. The amount does not admit of accurate appreciation, but it is considerable. It of course varies according to circumstances. In hypertrophy of the heart, the quantity is sometimes increased; as well as in ordinary cases of what are called determinations of blood to the head. Here, too large an amount is sent by the arterial vessels; but an equal accumulation may occur, if the return of the blood from the head by the veins be in any manner impeded, as when we stoop, or compress the veins of the neck by a tight cravat, or by keeping the head turned for a length of time. Congestion or accumulation of blood may therefore arise from very different causes.

-

Sir Astley Cooper1 found by experiment, that the vertebral arteries are more important vessels as regards the encephalon and its functions in certain animals, as the rabbit, than the carotids. The nervous power is lessened by tying them; and, in his experiments, the animals did not, in any case, survive the operation more than a fortnight. In the dog, he tied the carotids with little effect, but the ligature of the vertebrals had a great influence. The effect of the operation was to render the breathing immediately difficult and laborious; owing, in Sir Astley's opinion, to the supply of blood to the phrenic nerves, and the whole tractus respiratorius of Sir Charles Bell, being cut off. The animal became dull, and indisposed to make use of exertion; or to take food. Compression of the carotids and the vertebrals at the same moment, in the rabbit, destroyed the nervous functions immediately. This was effected by the application of the thumbs to both sides of the neck, the trachea remaining free from pressure. Respiration ceased entirely, with the exception of a few convulsive gasps. The same fact was evinced in a clearer and more satisfactory manner by the application of ligatures to the four vessels, all of which were tightened at the same instant. Stoppage of respiration and death immediately ensued.

The cerebral, like other arteries, are accompanied by branches of the great sympathetic. The researches of Purkinje, Volkmann, and Rainey, have shown the existence of a large number of nerves in connection with the encephalic and spinal arachnoid. They do not seem to communicate with the roots of the spinal nerves, but belong exclusively to the sympathetic. The encephalic veins are disposed as already described, terminating in sinuses formed by the dura mater, and conveying

1

Guy's Hospital Reports, i. 472, London, 1836.

? Müller's Archiv. für Anatomie, p. 281, Berlin, 1845.

* Art. Nervenphysiologie, Wagner's Handwörterbuch der Physiologie, 10te Lieferung, s. 494, Braunschweig, 1845.

4 Medico Chirurgical Transactions for the year 1845.

5 D. Brinton, Art. Serous and Synovial Membranes, in Cyclop. of Anat. and Physiol., Pt. xxxiv. p. 525, Lond., Jan. 1849.

11

10

Fig. 31.

Sinuses of the Base of the Skull.

1. Ophthalmic veins. 2. Cavernous the figure occupies the position of the pituitary gland in the sella turcica. 4. Inferior petrosal sinus. 5. Transverse or anterior occipital sinus. 6. Superior petrosal sinus. 7. Internal jugular pital sinuses. 10. Torcular Herophili. vein. 8. Foramen magnum. 9. Occi

sinus of one side. 3. Circular sinus:

11, 11. Lateral sinuses.

Fig. 32.

their blood to the heart by means of the lateral sinuses and internal jugulars; but of the peculiarities of the circulation in the encephalon, mention will be made in the appropriate place. No lymphatic vessels have been detected in the encephalon; yet, that absorbents exist there is proved by the dissection of apoplectic and paralytic individuals. In these cases, when blood has been effused, the red particles are gradually taken up, with a portion of the fibrinous part of the blood, leaving a cavity called an apoplectic cell, which is at the same time the evidence of previous extravasation and subsequent absorption.

The whole of the nervous system is well supplied with blood vessels. In the vesicular neurine of the nervous centres, the capillaries surround the ganglion cells or globules; and in the tubular they pass between the nerve-tubes, being connected at intervals by transverse branches.

When the skull of the new-born infant, which, at the fontanelles, consists of memhas received an injury, that exposes the brane only-or the head of any one who brain-is examined, two distinct movements are perceptible. One, which is generally obscure, is synchronous with the pulsation of the heart and arteries; the other, much more apparent, is connected with respiration, the organ seeming to sink at the time of inspiration, and to rise during expiration. This phenomenon is not confined to the cerebrum, but exists likewise in the cerebellum and spinal marrow. The motion of the encephalon, Capillary Net-work of Nervous synchronous with that of the heart, admits of easy explanation. It is owing to the pulsation of the circle of arteries at the base of the brain elevating the organ at each systole of the heart. The other movement is not so readily intelligible. It has been attributed to the resistance, experienced by the blood in its passage through the lungs during expiration, owing to which an accumulation of blood takes place in the right side of the heart; this extends to the veins and to the cerebral sinuses, and an augmentation of bulk is thus occasioned. We shall see hereafter, that one of the forces conceived to propel the blood along the vessels is atmospheric pressure. According to that view, the sinking down of the brain during inspiration is explicable: the blood is rapidly drawn to the heart; the quantity in the veins is consequently diminished; and sinking of the brain succeeds.

Centres.