_Protopathic sensibility_ is of a lower order than epicritic. It consists in the recognition of painful cutaneous stimuli and of extreme degrees of heat and cold. The fibres concerned are non-medullated and regenerate comparatively quickly after injury, so that protopathic sensibility is regained before epicritic.
_Epicritic sensibility_ is the most highly specialised and permits of the recognition of light touch, _e.g._, with a wisp of cotton wool, of fine differences of temperature, and of discriminating as separate the points of a pair of compa.s.ses 2 cm. apart. These sensations are carried by medullated nerve fibres, and are slow to return after injury to the nerves.
The sensory nerve fibres conveying these different impulses pa.s.s to the ganglionic cells of the posterior nerve roots. From each of these cells a process pa.s.ses into the cord and bifurcates into an ascending and a descending branch. In the cord the fibres rearrange themselves and pa.s.s to the brain by a double path. Those that convey sensations of pain and of temperature pa.s.s by the spino-thalamic route by way of the tract of Gowers and the fillet to the optic thalamus; those that are concerned with the muscular sense, the joint sense, and tactile discrimination pa.s.s up the posterior columns in the tracts of Goll and Burdach to the nuclei gracilis and cuneatus in the medulla, whence they pa.s.s to the optic thalamus.
From the cell station in the optic thalamus the fibres proceed to the _cortical sensory centres_, that for tactile sensation being situated in the post-central (ascending parietal) gyrus; that for muscular and stereognostic sense lying probably in the adjacent portions of the parietal lobe.
In a unilateral lesion of the cord, pain and the temperature sense may be disturbed in one limb, and motor power and tactile sensibility in the other, as the fibres that convey impressions of pain, and those that subserve the discrimination of temperature, pa.s.s up and decussate in the cord a few segments above their point of entrance.
[Ill.u.s.tration: FIG. 180.--Diagram of the Course of Motor and Sensory Nerve Fibres.]
#Effects of Lesions of the Motor and Sensory Mechanisms.#--Lesions of the _motor mechanism_ differ in their fundamental characters according as they affect the upper or the lower neurones. The signs also vary according as the affected area is _destroyed_ or merely _irritated_, say by the pressure of a tumour. Irritative lesions in general produce muscular spasms or convulsions, while destructive lesions cause paralysis. The essential differences in the effects of destructive lesions of upper and lower neurones may be indicated thus:--
_Upper Neurone Lesion._ _Lower Neurone Lesion._
Spastic paralysis of voluntary Flaccid paralysis of voluntary muscles. muscles.
No marked wasting of paralysed Marked wasting of paralysed muscles. muscles.
No reaction of degeneration. Reaction of degeneration.
Exaggeration of reflexes. Loss of reflexes.
Irritative lesions of the sensory mechanism cause numbness and tingling (paraesthesia); more extensive paralytic lesions produce anaesthesia, astereognosis, loss of muscle sense, loss of pain, or inability to distinguish temperature, according to the tracts that are affected.
_Lesions of the Upper Motor Neurone_ may occur in any part of its course. _Localised lesions of the motor cortex_ of an irritative kind, for example, a patch of meningitis, a tumour, meningeal haemorrhage, or a spicule of bone, produce spasms in those groups of muscles on the opposite side of the body that are supplied by the centres implicated--Jacksonian epilepsy. The cortical discharge may overflow into neighbouring centres and cause more widespread convulsive movements, or, if strong and long-continued, may even lead to general convulsions. Consciousness is usually lost before the whole of one side becomes implicated in the spasms; always before they spread to the opposite side. Contracture may occur in the muscles affected after the spasms cease.
If an area of the cortex is destroyed by the lesion, paralysis is produced of the corresponding muscles on the opposite side of the body. At first the paralysed muscles are flaccid, but spasticity soon develops. In some cortical lesions, for reasons not yet understood, the paralysis remains of the flaccid type. The seat and extent of the paralysis depend upon the area of the cortex destroyed. In rare cases the whole motor area is destroyed--_cortical hemiplegia_; more generally the lesion affects one or more groups of muscles, and occasionally all the muscles of one limb are paralysed--_cortical monoplegia_. Lesions are often both irritative and destructive, and lead to paralysis of one or more groups of muscles a.s.sociated with spasms and convulsions of the muscles governed by neighbouring areas of the cortex. Irritation or destruction of the sensory centres may also exist, giving rise to areas of paraesthesia and anaesthesia.
Lesions in the _centrum ovale_, which destroy the fibres proceeding from the overlying cortex, produce a corresponding spastic paralysis on the opposite side of the body. No irritative phenomena are a.s.sociated with such a sub-cortical lesion.
Lesions in the region of the _internal capsule_ often produce complete spastic hemiplegia of the opposite side of the body. When the posterior part of the capsule is involved, there are, in addition, hemianaesthesia and hemianopia, and sometimes disturbances of hearing, smell, and taste.
A lesion of the _crus_ may in like manner produce spastic hemiplegia and hemianaesthesia of the opposite side, often a.s.sociated with a lower neurone paralysis of the third and fourth nerves of the same side (crossed paralysis). The optic tract, which crosses the crus, may also be affected, and hemianopia result.
Lesions of the _corpora quadrigemina_ cause interference with the reaction of the pupil, disturbance of the functions of the oculo-motor nerve and of mastication, ataxia, and inco-ordination of the movements of the limbs.
The symptoms produced by lesions of the _pons and medulla_ vary according to the position of the lesion. If it is unilateral, there may be spastic hemiplegia and hemianaesthesia of the opposite side; if it is situated in the lower part of the pons or in the medulla, there is often also a lower neurone paralysis of one or more of the cranial nerves on the same side as the lesion (crossed paralysis). Paralysis of the external rectus of one eye and of the internal rectus of the other (conjugate paralysis) is frequently found in pontine, and in cortical and internal capsule lesions.
_Cerebellar_ lesions are a.s.sociated with special symptoms. In ataxia, there is inco-ordination of muscular movements, especially of the coa.r.s.e movements, such as walking. The gait becomes irregular and staggering, with a tendency to fall, sometimes to the side on which the lesion is situated, sometimes to the opposite side. In patients who cannot walk, ataxia may be tested by ordering repeated p.r.o.nation and supination of the forearm. Paresis or asthenia may be found in the trunk muscles, or evidenced by weakness of the grip, or drooping of the head to one side. Changes in muscle tone may arise and lead to exaggerated or decreased reflexes, often varying from day to day.
Vertigo and nystagmus may also be present, in addition to occipital headache and tenderness on percussion. When one lateral lobe is implicated, the symptoms are referred to the same side; when the median lobe is involved, they are bilateral, and there may be retraction of the neck with extension of the legs, probably as the result of the a.s.sociated internal hydrocephalus.
A unilateral lesion of the _spinal cord_ causes a lower neurone paralysis of the muscles supplied from the cord at the level of the lesion, with spastic paralysis of the muscles of the same side of the body supplied from a lower level of the cord. The sensory symptoms are variable. Typically there is some anaesthesia in the structures supplied from the damaged section of the cord--incomplete owing to the overlapping by other sensory nerves. Just above the lesion there is irritation of spinal nerves, and hyperaesthesia and pain referred to their distribution. On the same side below the lesion, there is a loss of epicritic, stereognostic and deep sensibility, and on the opposite side below the lesion, loss of the sense of pain and the discrimination between heat and cold. Ordinary tactile sensibility, which is governed by a double path, may or may not be lost on either side below the lesion.
#Other Special Centres.#--The cortical centres for _vision_ lie on the median surfaces of the occipital lobes in the neighbourhood of the calcarine fissure. Each half-vision centre--for there is one in each occipital lobe--receives the fibres from the same side of both retinae.
Destruction of one half-vision centre produces the condition known as _h.o.m.onymous hemianopia_, in which the medial (nasal) half of one visual field and the lateral (temporal) half of the other is affected, so that there is an inability to see objects situated on the side opposite to the lesion.
_Auditory impulses_ are received in the posterior part of the superior temporal convolution.
_Aphasia._--The use of language, spoken or written, as a means of expression depends upon the co-ordination of four different centres: the visual, the auditory, the graphic, and the articulatory. These are situated in different parts of the brain and are connected by sub-cortical a.s.sociation tracts, the main pathway of which lies in the vicinity of the upper end of the fissure of Sylvius. Marie has proved that aphasia results from lesions in this area.
The _olfactory_ and _gustatory_ centres are situated in the uncus close to the pituitary fossa.
Lesions of the frontal cortex anterior to the motor centres, even if extensive, may produce few or no symptoms, and in consequence this region has been called a "silent" area. Occasionally there results a change in temperament or intelligence, and the region is on this account supposed to be concerned with the higher psychical functions.
There is evidence that the pre-frontal cortex has a centre for the conscious initiation of movements, and that lesions produce "apraxia,"
_i.e._, inability to perform, or clumsiness in voluntarily performing fine movements such as touching the nose with the finger, though such movements may be perfectly carried out unintentionally. This centre is probably situated in the superior and middle left frontal convolutions in right-handed people. The fibres from the centre to the right motor area cross in the anterior part of the corpus callosum.
#Cerebral Localisation.#--The various parts of the brain can be localised in relation to the surface by various methods. That devised by Professor Chiene has been found reliable.
#Relation of Cerebral Centres to the Surface.#--Numerous attempts have been made to formulate rules for locating the different parts of the brain in relation to the surface of the head. The method devised by Chiene is free from many of the difficulties and fallacies common to most other methods, inasmuch as the results obtained do not depend upon making definite measurements in inches, or determining particular angles. Certain fixed and easily recognised bony landmarks--the glabella, the external occipital protuberance, the lateral angular process, and the root of the zygoma--are taken, and connected by lines, which are further subdivided--_always being bisected_. Figs.
179 and 181 explain the method. The head being shaved, a line (GO) is drawn along the vertex from the glabella (G) to the external occipital protuberance (O). This line is bisected in M, which const.i.tutes the "mid-point." The posterior half of the line MO is bisected in T, const.i.tuting the "three-quarters point," and the posterior half TO is bisected in S--"the seven-eighths point." The lateral angular process (E) is next connected to the root of the zygoma (P) by a line EP, and the root of the zygoma with the seven-eighths point by PS; the line EPS thus forms the base line. The lateral angular process is now joined to the three-quarters point by ET. The two segments of the base line EP and PS are bisected in N and R respectively, and these points connected with the mid-point (M) by lines NM and RM. These lines cut off a part of ET--AB, which is now bisected in C, and from C the line CD is drawn parallel to AM.
[Ill.u.s.tration: FIG. 181.--Chiene"s Method of Cerebral Localisation.]
In this way practically all the points of the brain which are wanted for operative purposes may be mapped out. Thus the quadrilateral s.p.a.ce MDCA contains the Rolandic area. MA represents the praecentral sulcus, and if it be trisected in K and L, these points will correspond to the origins of the superior and inferior frontal sulci. The pentagon ABRPN corresponds to the temporal lobe. The apex of the temporal lobe extends a little in front of N. The supra-marginal convolution lies in the triangle HBC. The angular gyrus is at B. A is over the anterior branch of the middle meningeal artery, and the bifurcation of the lateral or Sylvian fissure; AC follows the horizontal limb of the lateral fissure. The transverse or lateral sinus at its highest point touches the line PS at R (Fig. 181).
The _fissure of Rolando_ or _central sulcus_ may be marked out by taking a point half an inch behind the mid-point (M) (Fig. 181), and drawing a line downwards and forwards for a distance of about three and a half inches, at an angle of 67.5 with the line GO. The angle of 67.5 can be readily determined by folding a square piece of paper on itself so as to make a triangle. The angle at the fold equals 45. By folding the paper again upon itself in the same direction, the right angle of the paper is divided into four angles of 22.5 each. Three of these angles taken together make up the 67.5. If the straight edge of the paper be placed along the sagittal suture with the angle of folding over the upper end of the fissure of Rolando, the folded edge falls over the line of the fissure (Chiene).
[Ill.u.s.tration: FIG. 182.--To ill.u.s.trate the site of various operations on the skull.]
LUMBAR PUNCTURE
Quincke, in 1891, first suggested the withdrawal of cerebro-spinal fluid from the theca in the lumbar region, as a means of relieving excessive intra-cranial tension in tuberculous meningitis, and to obtain specimens of the fluid for diagnostic purposes. The scope of the procedure, both as a therapeutic and as a diagnostic measure, has since been widely extended.
_Technique._--The puncture may be made with the patient either lying on his left side, the spine being fully flexed by approximating the knees and shoulders; or sitting on the table with the knees drawn up and the body bent forward. The upper edge of the fourth lumbar spine is identified by drawing a horizontal line across the back at the level of the highest part of the iliac crests (Fig. 183). The s.p.a.ce between the fourth and fifth lumbar vertebrae being the widest, is that usually selected. The skin having been purified, an exploring needle, about three inches long, is introduced about half an inch below the fourth lumbar spine in the middle line, and pa.s.sed for about two inches in a direction forwards and slightly upwards. The needle usually encounters some resistance as it pierces the interspinous ligament, and then enters the sub-arachnoid s.p.a.ce. If bone is struck, the needle should be withdrawn and introduced at a different level. If the cerebro-spinal fluid does not escape at once, a stylet should be pa.s.sed through the needle to clear it of blood-clot or shreds of tissue. When the intra-thecal tension is normal, the fluid trickles away drop by drop, but if it is increased, as, for example, in meningitis, intra-cranial tumour, hydrocephalus, or uraemia, it may escape in a jet.
[Ill.u.s.tration: FIG. 183.--Localisation of site for introduction of needle in Lumbar Puncture.]
The _normal cerebro-spinal fluid_ is clear and colourless, has a specific gravity of 1004-1008, and contains a trace of serum globulin and alb.u.mose, some chlorides, and a substance which reduces Fehling"s solution. Microscopically, it may contain some large endothelial cells and a few lymphocytes, or may be entirely devoid of cells. It does not contain the ant.i.toxins and opsonins which are normally found in the plasma and lymph, hence the liability to infective meningitis after injuries and operations on the central nervous system. With a view to diminishing these risks, hexamine, which is excreted into the cerebro-spinal fluid, is administered for its antiseptic properties in cases of head injury and before intra-cranial operations.
_Diagnostic Puncture._--Examination of the fluid withdrawn has proved useful in diagnosis in cases of intra-cranial and intra-spinal haemorrhage, in various forms of meningitis, in cerebral abscess, and in some cases of cerebral tumour.
The first few drops should be discarded, as they may be stained with blood from the puncture, and about 5 c.c. collected in each of two sterile tubes. To determine whether blood in the fluid is due to the puncture or to a pre-existing intra-cranial or intra-thecal haemorrhage, the fluid should be centrifugalised; in the former case the supernatant fluid is clear and limpid, in the latter it retains a yellow tinge. In extra-dural haemorrhage there is no blood in the cerebro-spinal fluid.
In acute meningitis the fluid is turbid, and contains an excess of alb.u.min. Organisms also are present, such as the diplococcus intracellularis in acute cerebro-spinal meningitis; staphylococci, streptococci, and pneumococci, particularly in the intra-cranial complications of middle ear disease. In all cases of acute microbic infection, and especially in the suppurative forms, polynuclear leucocytes are found in the fluid; while in chronic affections, such as tubercle and syphilis, there is an excess of lymphocytes (Purves Stewart). The detection of the tubercle bacillus is confirmatory of a diagnosis of tuberculous meningitis, but, as it is often difficult to find, its absence does not negative this diagnosis. In tuberculous meningitis the clot which forms floats in the centre of the fluid, and is translucent, grey, and flaky; in the pyogenic forms it is yellow, and sticks to the side of the vessel.
In a few cases of malignant tumour of the spinal cord and its membranes, characteristic cells have been found in the fluid after centrifugalising.
In uraemia there is a diminution of chlorides, and an increase of phosphates and sulphates.
The Wa.s.serman test is sometimes positive in the cerebro-spinal fluid, when it is negative in the blood.
_Therapeutic Puncture._--In certain cases of cerebral tumour, and of tuberculous meningitis a.s.sociated with an excessive quant.i.ty of fluid in the arachno-pial s.p.a.ce, temporary relief of such symptoms of increased intra-cranial tension as headache, vertigo, blindness, or coma, has followed the withdrawal of from 30 to 40 c.cm. of the fluid.
Terrier and others have found this measure useful in relieving pain in the head, delirium, and even coma, in cases of basal fracture.
Carriere has found it beneficial in some cases of uraemia. The quant.i.ty withdrawn must not exceed 40 c.cm., lest the ventricles be emptied and pressure be exerted directly on the basal ganglia (Tuffier). In a number of cases sudden death has followed the withdrawal of cerebro-spinal fluid.
This route is sometimes selected for the induction of spinal anaesthesia, and for the injection of ant.i.toxin in cases of teta.n.u.s.
HEAD INJURIES