Page:The New International Encyclopædia 1st ed. v. 14.djvu/425

* NERVOUS SYSTEM AND BRAIN. 375 NERVOUS SYSTEM AND BRAIN. terminations. Among these may be mentioned ilerkel's iastxellcii, or touch cells, the tactile corpuscles of Meissner, and the Pacinian bodies. In tendons and in muscle, sensory nerve fibres, after losing their medullary sheaths, divide into minute fibrils which are often studded with irregular exjiansions. In gland tissue, nerve fibres usually end as fine fibrils, which pass to the epithelial cells. PlIY.SIOLOGY OF THE SplNAL CoRD AND SplNAL Xekves. The sensory nerve endings serve as the receptive apparatus by means of which e.ternal stimuli may induce a nervous impulse. The motor nerve endings serve as organs of distribu- tion through which an impulse may so aflect a muscle as to cause a contraction and consequent muscular action. The nerves themselves appar- ently serve merely as a conductive apparatus for transmitting the sensory impulses from the peri- phery to the spinal cord and the motor impulses from the cord to the muscles. The spinal cord must be considered as subserving several quite dillerent though related i)hysiological purposes, of which the following are the most important: (I) Its gray matter acts as a reflex centre, i.e. a centre in which an impulse brought to it by the fibres of the posterior nerve root (usually a sensory impulse) can determine an impulse pass- ing out in the fibres of an anterior nerve root (usually a motor impulse). In this way a motion is produced by an external stimulus without any involvement of consciousness. A simple reflex is one in which the impulse passes directly from the posterior root fibres to the cells of the an- terior horns. A compound reflex is one in which the impulse passes through another neurone sit- uated within the cord in passing from the pos- terior root fibre to the anterior horn cell. (2) An automatic centre. By an automatic action is meant one which apparenlhi occurs without any determining external stimulus, e.g. the rhyth- mical action of the heart or the contraction and expansion of the arteries. (3) A relay station in the transmission of impulses from and to the higher centres. This takes place through the fibre tracts of the cord and has been already re- ferred to in connection with the description of those tracts. (4) A conduction path. This also has been noted in connection with the de- scription of the fibre tracts of the cord. THE BRAIN. By this term is usually meant all that portion of the cerebro-spinal axis contained in the cranial cavity. The human brain is larger and heavier rela- tively to the size and weight of the body than the brain of any other animal, with the pos- sible exception of some of the snuiUcst birds and mammals. It is ^arger and heavier abso- lutely than that of any other animal except the elephant and larger whales. It is largest in the white races, smallest in some of the Central African savages. It is larger in the male than in the female. The average weight of the adult male brain of the white races of Kurope and America has been estimated at from 40 to 50 ounces, of the female at from 4.S to 4.5 ounces. The brain is much larger relatively to the size of the liody in the infant than in the adult, the brain of the new-born child weighing on an average from 10 to 12 ounces. The brain in- creases in weight up to middle life. After sixty it usually decreases sumeuhat in weight. It is a common idea that large brains are apt to be associated with unusual intellectual development. .Such is not, however, the case. While the brains of some men of great intellectual attainments have proved to be above the average in weight — Cuvier, 64 ounces; Abercrombie, (i3 ovmces; Goodsir, 57 ounces — the brains of other men ap- parently equally intellectual have not been above the average in weight, and Thurman reports a case of a male epileptic whose brain weighed 62 ounces, and liicknill one from a similar case which weighed O414 ounces. Brains weighing over 60 ounces have also been observed in the insane. The brain may be subdivided into the medulla oblongata, the pons Varolii, the midbrain, the cerebellum, the basal ganglia, and the cerebral hemispheres. The medulla oblongata is the continuation upward of the spinal cord and extends from the lower limit of the pyramidal decussation below, to the lower margin of the pons above. The length of the me- dulla is about an inch, and its diam- eters, which at its lower portion cor- respond to those of the cord, increase from below upward. Externally the me- dulla shows the continuation u p - ward of the ante- rior and of the pos- terior fissures of the cord. These fis- sures become more shallow as they as- Pf^py •litOi.jKd.cer OlitMy. -F(ss.dec MEDULLA 0BLON{J.TA A.ND PONB VAROLII. Pon. Vsr., pons Varolii: .Vid., ,, ^ . ped. rer., middle peduacle of cere- cend, the posterior helium: Oliv. txl.v.. olivary body; fissure disappear- Fiss.dec, pyramidal derusnation ; ino- nt nbnnf the '"<"'■ "'''•• medulla oblongata; .", ,, y ?, F.y- IVrs.. anterior pyramids, middle of the me- dulla, where the central canal opens into the fourth ventricle. On either side of the anterior fissure is a prominence caused by the anterior pyramid, and to the outer side of the pyramid the bulging of the olivary body may be seen. The antero-lateral surface of the medulla is also marked by the exit of the sixth to the twelfth (inclusive) cranial nerves. The posterior surface shows two prominences on cither side, one next the pos- terior fissure known as the clava, caused by the nucleus gracilis or nucleus of the column of Goll : the other just to the outer side of the clava, due to the nucleus cuncatus or nucleus of the column of Burdaeh. The internal structure of the medulla shows considerable resemblance to that of the cord. This is especially true of the lower part of the medulla, the structures of which are directly continuous with those of the cord. The fibre tracts of the cord, however, as- sume, in the medulla, new directions, and in do- ing so break up the formation of the gray matter. This, together with the appearance of some new masses of gray matter and some new fibre bundles, is the main factor in determining the difTerence in structure between cord and medulla. The internal structure of the medulla can be best understood by tracing into it the structures of the spinal cord. At the junction of medulla and