Manual of Surgery Volume Second: Extremities—Head—Neck. Sixth Edition.

by Alexis Thomson; Alexander Miles

The cerebro-spinal fluid is secreted by the choroid plexuses and fills the cerebral ventricles, the central canal of the cord, the sub-dural and sub-arachnoid spaces, and the sheaths of the intra-cerebral blood vessels. At the base of the brain, particularly in the posterior fossa, the sub-arachnoid space is wider than elsewhere, forming “cisterns” filled with cerebro-spinal fluid which supports the cerebral structures. Through the foramen of Magendie in the roof of the fourth ventricle the sub-arachnoid fluid of the cranial cavity communicates with that of the vertebral canal.

Although it differs in its chemical constitution from true lymph, the cerebro-spinal fluid seems to functionate as lymph, in addition to acting as a lubricating agent, and playing a part in regulating the vascular supply of the brain. In cases of cerebral hæmorrhage, abscess, tumour, or depressed fracture, room is made up to a certain point for the extraneous matter by displacement of cerebro-spinal fluid.

Vascular supply.—The free anastomosis between the vessels entering into the formation of the circulus arteriosus (circle of Willis) ensures an abundant supply of blood to the brain. The larger arteries run in the sub-arachnoid space and give off branches which ramify in the pia mater before entering the cerebral substance. Within the brain, each artery being more or less terminal, there is no free anastomosis between adjacent vessels, with the result that if any individual artery is obstructed the vitality of the area supplied by it is seriously impaired. The venous arrangements are also peculiar in that the veins are thin-walled and valveless, and open into the rigid, incompressible sinuses which run between the layers of the dura mater. Most of the blood passes to the internal jugular vein, and any increase in the pressure of this vessel is immediately transmitted back to the cerebral veins. As the blood vessels project into a rigid case filled with incompressible material, and as the total volume of blood in the brain is constant (Munro and Kelly), any alteration in the supply of blood to the cerebral tissue must be due to an increased velocity of flow, and this in turn depends upon changes in the aortic and vena cava pressure. Thus, if the aortic pressure rises, more blood will enter the cerebral vessels and will move along more rapidly; while if the pressure in the vena cava rises there is obstruction to the passage of blood in the arteries and diminished velocity of flow. The ebb and flow of cerebro-spinal fluid in and out of the spinal canal may also help to control the pressure.

Nerve Elements.—The nervous system is composed of a multitude of units, called neurones, each neurone consisting of a nucleated cell, with branching protoplasmic processes or dendrites and one axis-cylinder or axon. The nutrition of an axis cylinder depends on its continuity with a living cell. If the cell dies, the axis cylinder degenerates. If the axis cylinder is severed at any point, it degenerates beyond that point, and the nucleus of the nerve-cell disintegrates—chromatolysis.

The axis cylinder of one cell ends in a number of fine filaments which arborise around another nerve-cell, thus bringing it into physiological, if not anatomical, relationship with the first cell. The termination is called a cell-station or synapsis. In this way the various sections of the nervous system are kept in association with one another and with the rest of the body.

Motor Functions and Mechanism.—The nerve centres, which together make up the motor area, and govern the voluntary muscular movements of the body, are situated in the grey matter of the præcentral or ascending frontal gyrus, and of the frontal aspect of the central sulcus (fissure of Rolando). The upper limit of the motor area reaches on to the mesial aspect of the paracentral lobule, and the lower limit stops short of the lateral cerebral fissure (fissure of Sylvius) ([Fig. 179]).

Each group of muscles has its own regulating centre, the size of the area representing any group depending upon the character and complexity of the movements performed by the muscles, rather than upon the amount of muscular tissue that is governed by the centre—for example, the centre for the mouth, tongue, and vocal cords is larger than that for the muscles of the trunk.

The motor centres have been localised on the surface of the brain with approximate accuracy. For example, above the superior genu of the præcentral gyrus, the centres governing the hip, knee, and toes are grouped; opposite the genu are the centres for the movements of the trunk; between the superior and middle genua lie the centres for the upper extremity; opposite the middle genu, those for the neck, and below it, those for the face, jaws, and tongue, pharynx and larynx.