Page:The American Cyclopædia (1879) Volume III.djvu/193

 BRAIN 1ST ever, is less distinct in the fibres of the brain than in those of the nervous trunks, and in some instances appears to be altogether wanting. The fibres of the brain average -nj-B-j-j f an s Fio. 1. Nerve Fibres of the Brain. inch in diameter, presenting at some points a swollen appearance ; they do not communicate with each other like the vessels, nor divide into smaller fibres, but continue unbroken from their origin to their final distribution, inoscu- lating only at their terminal loops. The gelat- inous or gray fibres seem to be solid, flattened, transparent filaments, varying in diameter from 60*00 to 3-jnnr of an inch ; the mode of their con- nection with the elements of the nervous cen- tres is unknown. The essential elements of the vesicular or gray nervous matter are cells, or vesicles, containing nuclei and nucleoli; they are dark, generally globular, but at times very irregular and variously elongated, enclosing a grayish granular substance, and sometimes pig- ment granules ; they vary in size from j-yVv to j^j of an inch in diameter ; among the largest of these are the caudate, so called from the ir- regular tail-like process extending from them. Fia. 2. Norvo Colls of tho Brain. The nerve vesicles are imbedded in a soft gran- ular matrix in the brain. The nervous centres exhibit tho union of these two forms of matter, more widely separated in the brain than in the smaller ganglia; indeed, the cerebral hemi- spheres are composed internally of fibrous mat- ter exclusively, surrounded by a layer of the gray vesicular substance, into which the fibres are also prolonged. The tubular fibres seem to be capable of regeneration to a certain extent ; if the nerve be divided, but the ends not sep- arated, union may take place, and the nerve resume its office ; even when a portion is ex- cised, it appears that true nerve fibres, in small- er number than in the nerve itself, may be de- veloped in the uniting substance, as shown by partial restoration of function and microscopic examination. When a portion of the brain is removed by accident or design, its place is sup- plied by new substance ; but whether this be true cerebral substance or not has not been satisfactorily determined. The white fibres may be distinguished, according to their phys- iological office, into three kinds efferent or motor, afferent or sensitive, and commissural or connecting. Henle suggests that there may be a fourth series, associated with the ope- rations of thought. Of the mode in which the afferent nerves terminate, and the motor nerves commence in the central organs, it may bo said that three principal modes have been ascertained, in which there is an actual conti- nuity from one form of nerve tissue to the other : a globular unipolar cell may give out a single prolongation, which becomes a fibre ; or a nerve cell may be found in the course of a tube, with each extremity prolonged into a fibre ; or some of the radiating prolongations of the caudate cells may become continuous with the axis cylinders of nerve tubes, or inosculate with those of other caudate cells. A curious cir- cumstance in connection with the gray matter is the large quantity of pigment or coloring sub- stance in it, apparently forming one of its es- sential constituents, as it is everywhere pres- ent, though in some situations more abundant- ly than in others ; it has been asserted that this bears a close resemblance to the coloring mat- ter of the blood, and if so, it is a fact of great interest to physicians, who can avail themselves of the restorative properties of iron in cerebral diseases, improving tho quality of the nutrient blood by increasing the quantity of the red globules. The central column or spine of the vertebrate skeleton encloses in its canal the spinal cord ; and the cranium, which is a series of modified and expanded vertebne, protects the continuation of the cord and its expansion into an aggregate of gangliform swellings, the brain or encephalon. The brain is enclosed in three membranes, or meninges, continuous with those of the spinal cord, which will be described un- der that head. From without inward, these membranes are thodtira mater, arachnoid, and pia mater. The term mater (fify-i/p, mother) originated with the Arabians, who considered these membranes as the parents of all others in the body. The dura mater is a membrane of white fibrous tissue, strong, flexible, but not elastic; its fibres are arranged on different