Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

MID

Online Encyclopedia
Originally appearing in Volume V25, Page 670 of the 1911 Encyclopedia Britannica.
» Make a correction to this article.
» Add information or comments to this article.
Spread the word: del.icio.us del.icio.us it!

MID .VENTRAL LA 14A (From D. J. See also:

Cunningham in Cunningham's See also:Text-See also:Book of See also:Anatomy.) Flo. 5.—Schema of a' Transverse See also:Section through the See also:Early Neural See also:Tube (See also:Young). The See also:left See also:side of the section shows an earlier See also:stage than the right side. of the See also:canal and shoot out See also:long processes—the axons. The permanent central canal of the See also:cord was formerly said only to represent the ventral end of the large embryonic canal, the dorsal See also:part being converted into a slit by the See also:gradual closing in of its lateral walls, thus forming the postero-median fissure. A, See also:Robinson, however, does not believe that the posterior fissure is any remnant of the central canal, and there are many points which See also:bear out his contention (Studies in Anatomy, See also:Owens See also:College, 1891). The most See also:modern view (1908) is that the fissure is formed partly by an infolding and partly from the See also:original central canal. The antero-median fissure is caused by the ventral part of the cord growing on each side, but not in the mid-See also:line where no germinal cells are. The anterior See also:nerve roots are formed by the axons of the ,neuroblasts in the developing anterior cornea, but the posterior grow into the cord from the posterior See also:root ganglia (see NERVE: See also:Spinal), and, as they grow, See also:form the columns of Goll and Burdach. That part of the See also:grey See also:matter from which the ventral, anterior or motor nerve roots rise is known as the basal lamina of the cord, while the more dorsal part into which the posterior nerve roots enter is the alar lamina.

These parts are important in comparing the See also:

morphology of the spinal cord with that of the See also:brain. In the embryo up to the fifth See also:month there is little difference in the See also:appearance of the grey and See also:white matter of the cord, but at that See also:time the See also:fibres in the columns of Burdach acquire their medullary sheaths or white substance of See also:Schwann, the fatty matter of which is probably abstracted from the See also:blood. Very soon after these the basis bundles myelenate and then, in the See also:sixth month, the columns of Goll. Next follow the See also:direct cerebellar tracts and, in the latter See also:half of the eignch month the tracts of Gowers, while the fibres of the pyramidal, and Lissauer's tracts do. not gain their medullary sheaths, until just before or after See also:birth. At first the spinal cord exends as far as the last mesodermal somite, but neuroblasts are only formed as far as the first See also:coccygeal somite, so that behind that the cord is non-See also:nervous and degenerates later into the filum terminate. After the See also:fourth month the nervous portion grows more slowly than the See also:rest of the See also:body and so the long cauda equina and fdum terminate are produced. At birth the See also:lower limit of the cord is opposite the third lumbar vertebra, but in See also:post-See also:natal development it recedes still farther to the, lower level of the first. For further details see See also:Quain's Anatomy, vol. i. (See also:London, 1908); J. P. McMurrich, Development of the Human Body (1906). Most modern descriptions are founded on the writings of W.

His, references to which and to other literature will be found on p. 463 of McM urrich's book. CORP See also:

Comparative Anatomy.—In the See also:Amphioxus. there is little difference between the spinal cord and the brain; the former reaches the whole length of the body and is of See also:uniform calibre. It encloses a central canal from which, a dorsal fissure extends to the See also:surface of the cord and it is composed of nerve fibres and nerve cells; most of the latter being grouped See also:round the central canal or neurocoele, as they are in the human embryo. Some very large multipolar ganglion cells are See also:present, and there are also large fibres known as See also:giant fibres, the See also:function of. which is. not clear. When the See also:reptiles are reached the cord shows slight enlargements in the regions of the limbs and these become more marked in birds and mammals. In the lumbar region of birds the dorsal columns diverge and open up the central. canal, converting it into a See also:diamond-shaped space which is only, roofed over by the membranes of the cord, and is known as the. sinus rhomboidalis. In all these lower vertebrates except the Anura (frogs and toads), the cord fills the whole length of the spinal canal, but in the higher mammals (See also:Primates, See also:Chiroptera and Insectivore) it grows less rapidly, and so the posterior part of the canal contains the cauda equina within its. sheath of dura mater. In mammals below the anthropoid apes there. are no direct, pyramidal tracts in the cord, since the decussation of the pyramids in the medulla is See also:complete. Moreover, the crossed tracts vary very much in their proportional See also:size to the rest of the cord in different animals. In See also:man, for example, they form 11.87 % of the See also:total See also:cross See also:area of the cord, in the See also:cat 7.76%, in the See also:rabbit 5.3 %, in the See also:guinea-See also:pig 3 %, and in the See also:mouse 1.14 %. In the See also:frog no pyramidal See also:tract is found.

It is obvious, there-fore, that in the lower vertebrates the motor fibres of the cord are not so completely gathered into definite tracts as they are in man. A See also:

good See also:deal of See also:interest has lately been taken in a nerve bundle which in the lower vertebrates runs through the centre of the central canal of the cord, and takes its origin in the optic reflex cells in See also:close relation to the posterior commissure of the brain. More posteriorly (caudad) it probably acquires a connexion with the motor cells of the cord and is looked upon as a means by which the muscles can be made to actively See also:respond to the stimulus of See also:light. " It is known as Reissner'sfibre, and its morphology and See also:physiology have been studied most carefully in cyclostomes and fishes. It is said to be present in the mouse, but hitherto no trace of it has been found in man. It was discovered in 186o, but for See also:forty years has been looked upon as an artifact. See P. E. See also:Sargent, " Optic Reflex Apparatus of Vertebrates," See also:Bull. See also:Mus. Comp. Zool.

Harvard, vol. xlv. No. 3 (See also:

July, 1904) ; also for See also:general details R, Wiedersheim, Comparative Anatomy of Vertebrates (London, 1907)";Lenhosrsek, Bau See also:des Nervensystem.i (1895). (F. G.

End of Article: MID

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
MICROTOMY (Gr. -mµ17; r ivsu', to cut) 		[next]
MIDAS

Site © 2006 - Net Industries