to the medulla oblongata, as it is likewise in the Cetacea. In most Rodentia the myelon terminates in the lumbar region, but in the rabbit it extends a little way into the sacrum. In the mouse the relative proportion of the myelon to the brain is as 22 to 100. In the Cetacea and Sirenia, the myelon presents only the anterior enlargement, which is very near the brain, and is remarkable for the close aggregation of the origins of the nerves from that part. The myelon is closely invested by the dura mater, which is directly perforated by the nerves, and the sheath terminates at the pointed end of the myelon, not being continued as such, over the cauda equina.' The myelon is small in proportion to the size of the body, shows the central canal, and, Hunter remarks, is more fibrous than in other animals; when an attempt is made to break it longitudinally, it tears with a fibrous appearance, but transversely it breaks irregularly."1

6

2

In the Elephant the dura mater surrounds the myelon less closely than in the Cetacea, and the roots of the nerves have a longer course within the sheath. In the Giraffe I found the myelon closely invested by the dura mater, which was thinner on the dorsal than on the ventral side: it is chiefly remarkable for the length of the cervical portion, which from the corpora pyramidalia to the pectoral or brachial enlargement measured four feet three inches. The elongation of this part during fœtal developement proceeding by uniform interstitial addition, the roots of the nerves become equally separated from each other; and, as the lowest filament of one root was not further removed from the highest of the next below, than this from the succeeding filament of the same root, such filaments were extended over an unusual space of the myelon; the root of the third cervical coming from a tract of not less than six inches in length: the contrast between the cervical myelon of the Porpoise and Giraffe in this respect is striking.

With the singular exceptions of the Echidna, Hedgehog, and certain bats, the mass of the myelon bears a direct ratio to that of the body throughout the Mammalian series, and its structure is essentially the same. In the adult human male it a little exceeds an ounce in weight: its tissue is firmer than that of the brain. As in all Vertebrates, the ventral and dorsal surfaces are respectively divided into equal moieties by a longitudinal fissure, of which the dorsal is deepest, and, in the Mammalia, closest. In Man, the interfissural plate of pia mater can be shown to be a fold in the ventral (anterior) fissure, fig. 40, a, but is confluent as a XCIV. p. 374.

1

2 XCVII'.

single delicate layer of vascular tissue in the dorsal (posterior) one, ib. c. A layer of white neurine accompanies the ventral fold, which, when withdrawn, shows the fissure to be closed by such layer, perforated by numerous holes for capillaries: its fibres are transverse and form the white myelonal commissure.' The depth of the ventral fissure is greatest at the pectoral enlargement of the myelon, and gradually diminishes towards the cauda equina.' The deeper dorsal fissure penetrates fully one-half of the dorsoventral diameter of the myelon through the greater part of its course, but becomes shallower in the lumbar region: it is bounded by a layer of grey neurine, connecting the same tissue in each lateral moiety of the myelon, which layer forms the grey myelonal commissure.'

In the developement of the myelon, as of the encephalon, the central part contains a fluid which is reduced by the endogenous

Transverse section of the human myelon, close to the third
and fourth cervical nerves.
XVIII".

growth of neurine, on approaching maturity; it remains in the myelon, as its 'canal,' which is obvious in the cold-blooded Vertebrates,' and is exposed, in birds, as the ventricle of the pelvic enlargement,' as it is in the fourth ventricle' of all Vertebrates, where it bears the name of 'calamus scriptorius' in anthropotomy. The myelonal canal is more obvious in lower mam

3

Magnified ten diameters. mals2 than in Man, and in the fœtus than in the adult; in whom, whilst unobliterated, it is surrounded, like the more obvious myelonal canal in Reptiles, by the grey commissural neurine. The canal is lined by ciliate cells. The lateral columns of this tissue, united by the commissure, are thicker but less peripherally extended in the ventral, g, than in the dorsal, h, portions of the myelon. In transverse section the grey neurine resembles a comma, the concavity of which is directed outward, the head, fig. 40, g, is surrounded by the peripheral white neurine, and the tail, ib. h, i, is produced to the issue of the dorsal (posterior) nerve-roots, ib. k. The proportions of the grey and

1 vol. i. pp. 272. 296.

2

xx. vol. iii. p. 43, no. 1362.

8 XVIII".

Of

1

3

41

A

P

A

white neurine vary in different parts of the myelon. In fig. 41, 1 is a section at the fore (upper) part of the pectoral enlargement, the head of the comma is small, the tail narrow: in the middle of the enlargement, section 2, the head is larger, with more distinct processes, the tail is thicker. In the dorsal region, sections 3, the grey matter is more reduced than in the neck. In the lumbar region, sections, it again expands, the head shows the stellar character, is fenced off from the ventral periphery by a smaller extent of white neurine; the tail is thicker, but here becomes shorter and seems not to reach the dorsal surface. Near the termination of the myelon the comma-shape is lost, and the grey neurine reduced to a subcylindrical tract, slightly notched laterally and surrounded, save at the commissure, by the white neurine. this tissue the largest proportion exists in the cervical part of the myelon and its enlargement, where the small columns called 'posterior pyramids' are continued from the dorsal part of the medulla oblongata, contracting to a point, near the end of the brachial enlargements, and there allowing the proper dorsal (posterior) columns of the myelon to come into contact at the posterior fissure. The difference in the proportions of white and grey neurine in the ventral and dorsal tracts of the myelon coincides with the different nervous endowments of the pectoral and pelvic limbs: in the former volition and sensation are greatest; in the latter reflex actions with diminished sensibility: the exercise of the arms and hands induces more calls upon cerebral action, that of the legs Transverse sections of the and feet operates more exclusively through physical changes of the lumbar part of the myelon itself: hence, therefore, the need of a greater proportion of the reproductive or grey tissue. Numerous multi-caudate vesicles are present in the grey neurine, and linear tracts are continued from the major part of its periphery, as seen in transverse section, towards that of the myelon, accompanied by capillary vessels which enter the pia mater.

5

human Myelon.

A. Anterior or ve ntral.
P. Posterior or dorsal.'

The proportion of the neural canal to the myelon varies in

different mammals: it is greatest in the Cetacea, Sirenia and Seal-tribe, the space between the myelon and neural arches being occupied by blood vessels, which, in those aquatic orders, are chiefly arterial plexuses. In land-mammals and Man the veins pre

dominate, having more or less of the character of sinuses, as shown in the section of the lumbar vertebra, fig. 42, where the communication of the perineural' veins, d, with those of the tissue of the vertebral centrum, is shown. But the most constant fluid external to the myelon is that which has been called cerebro-spinal.' In the dorsal region of the neural canal, in Man, the position of this fluid is shown in fig. 43, where c is the myelon, with its pia mater and arachnoid, m the dorsal or posterior septum, n the nerve-roots, and s s the sub- or ent-arachnoid space. The use of the uniform support and defence afforded by the interposition of this fluid between the myelon and the hard walls of the neural canal is obvious.1

44

The arachnoid is disposed about the myelon, as about the brain, after the manner of the serous membranes; it consists of an exterior or parietal layer' reflected upon the myelon to form the internal or myelonal' layer. If a section be made through a pair of nerve-roots, those e.g. of the fifth cervical, fig. 44, the arachnoid is seen to be continued as a loose sheath, about the inter-neural part of the root, n n, and is reflected so as to form small culs-de-sac, at the orifices of emergence.

af

Transverse section of the myelon and its membranes across the roots of the fifth cervical nerves.

In Man the myelon is loosely invested by the dura mater,' to which it is attached by

1 XIX". In which the effects of the removal of this fluid in the Dog are described.

processes of the arachnoid called ligamentum denticulatum,' and the nerve-roots.

§ 204. Encephalon, its primary divisions.-The encephalon, or brain, of Mammals,

like that of lower Vertebrates, Turtle, fig. 45 (vol. i., Shark, fig. 187,

45

d

R

four primary seg

Brain of a Turtle (Chelone), side view.

ments or divisions, indicated by as many superincumbent, originally vesicular, masses, or pairs of masses; but consisting, not only of those, but of tracts of the myelencephalic columns from which those masses are successively developed.

The hindmost division, or 'epencephalon,' fig. 46, c, consists of the enlarging parts of the myelencephalic columns, a, called medulla oblongata,' of the superincumbent mass, c, originally a pair in the human fœtus (fig. 47, c), called 'cerebellum,' and of a transverse commissure of that body, called

'tuber annulare' or 'pons varolii,' p; the three parts, so named in

anthropotomy, are subordinate elements of one and the same primary division of the encephalon.1

The next division includes the parts of the myelencephalic columns which support, and from which are developed, the optic lobes, o: it is the 'mesencephalon,' figs. 45, 46 and 47,0. With the columnar elements are the parts called the fillet,' and processus a cerebello ad testes' in anthropotomy, including the 'third ventricle' and its prolongations into the vascular appendages

The severance of the 'pons,' and raising it, in association with parts of another segment, to the rank of a distinct primary division as mesocephalon,' and the severance of the medulla oblongata' from the cerebellum, as a co-equal division, called 'metencephalon,' indicate the warping of the judgment through habitual contemplation of the characteristically modified and developed parts of the human brain.