134 PSYCHOLOGY The Cerebral Hemispheres.— In man the cerebral hemi- spheres form the largest part of the central nervous system. Fig. 34 is a view of the hemispheres from above. They present an ovoid surface which corresponds to the inner surface of the vault of the cranium. The two hemispheres are separated from each other by the longitudinal fissure (fissura longitudinalis) which runs from the frontal pole to the occipital pole. A deep fold of the dura mater dips into the longitudinal fissure called the falx cerebri (not shown). It will be seen that when viewed from above the cerebral hemispheres completely obscure all other cranial structures. The cerebellum lies immediately under the occipital pole. There are three surfaces of the hemispheres: ( 1 ) the convex surface as seen from above (Fig. 34) or the side (Fig. 35), (2) the mesial surface, which can be seen only by pulling the two hemispheres apart (Fig. 33). The mesial surfaces bound the longitudinal fissure, or rather the falx cerebri lies be- tween the two mesial surfaces. (3) The basal surface shown In addition to the structures described each hemisphere in- cludes a convoluted and infolded pallium or mantle divided into lobes and gyri. The pallium consists of the gray outside surface called the cortex. The white matter of the cerebrum lies below the cortex. Only about one-third of the cortex is visible; the pther two-thirds is found in the walls and floors of the sulci and fissures. The folds of the cortex are called gyri or convolutions. They are separated from one another by sulci or by the deeper furrows called fissures. The outside surface of each hemisphere (Fig. 35) (pallium) is divided into lobes; the frontal (lobus frontalis), parietal (lobus parietalis), occipital (lobus occipi- talis), and temporal (lobus temporalis). The central lobe (in- sula) is concealed.
•The caudate and lenticular nuclei and the internal capsule (and the lateral ventricle), which we have already identified belong to and should be [escribed with the cerebral hemispheres. We considered them with the more udal structures so that we could describe continuous and adjoining pa>rts beginning with the cord and ending with the corpus callosum. ' NEURO-PHYSIOLOGICAL BASIS OF ACTION 135 In order to work out these divisions, first locate the great longitudinal fissure (Fig. 34, 1) separating the two hemispheres, then the fissure of Sylvius (fissura cerebri lateralis Sylvii), which is a lateral fissure, shown in Fig. 35, 4. The fissure of Eolando [sulcus centralis (Rolandi) ] begins above, near the high- est point of the hemisphere and courses outward and downward over the lateral surface of the brain (Fig. 35, 3) to the horizontal v 6 FIG. 35. — View of the left side of the adult human brain. 1, cerebellum; 2, parieto- occipital fissure (fissura parieto-occipitalis); 3, fissure of Rolando (sulcus centralis (Rolandi)); 4, fissure of Sylvius (fissura cerebri lateralis (Sylvii)); 5, pons (Varoli); 6, spinal cord (medulla spinalis).
fissure of Sylvius, which it rarely joins. The parieto-occipitalis (fissura parieto-occipitalis) fissure can be seen on the mesial sur- face (Fig. 33, 14). It runs from a cleft shown on the extended convex surface (Fig. 34, 13) downward and a little forward to join the calcarine fissure (fissura calcarina) (Fig. 33, 17). These two fissures enclose a wedge-shaped portion of the mesial cortex called the cuneus — an important visual station. Having located 136 PSYCHOLOGY these more important fissures, we can divide the pallium into its lobes. (1) The frontal lobe has a convex, lateral and basal sur- face. The convex surface begins at the frontal pole (polus fron- talis) and is bounded posteriorly by the fissure of Rolando. The mesial boundary is not well marked. The basal portion is shown in Fig. 30. Lying on its surface are to be seen the olfactory bulbs (bulbus olfactorius) and tracts (tractus olfactorius). (2) The parietal lobe lies behind the frontal in front of the occipital lobe and above the temporal. It is bounded frontally by the fissure of Rolando, and laterally by the fissure of Sylvius. Posteriorly it has no natural boundary, but a line drUwn from the end of the fissure of Sylvius to the cleft of the parieto-occipital fissure will give a convenient boundary. Its mesial boundary is the parieto- occipitalis. (3) The mesial surface of the occipital lobe begins at this fissure and extends to the occipital pole (polus occipitalis). This lobe has a basal surface (that immediately above the cere- bellum) and a lateral convex surface. (4) The temporal lobe lies below the fissure of Sylvius (a portion is concealed in the fissure of Sylvius). It has a convex surface, shown in Fig. 35, and a basal surface. (5) The central lobe (insula) is concealed. It lies in the floor of the fissure of Sylvius. It can be exposed by gently opening out the fissure. The location of the insula can It is in definite parts of the cortical surface of these lobes that the various ascending projection fibers coming from the cord and brain stem, which are soon to be described, find their ter- minations, thus giving the nerves of special senses — visual, audi- tory and olfactory, etc. — a definite cortical representation. We shall find further that cell bodies lying in the cortex give rise to axones which form descending or returning pathways to the lower structures. To make clear this architecture we must turn to a study of the internal structure of the brain and spinal cord. We shall first consider the make-up or composition of the peripheral spinal and cranial nerves and then examine into the connections of these nerves with the internal structures of the cord and brain.
THE PERIPHERAL NEURONES OF THE CORD AND BRAIN. The Peripheral Neurones of the Cord.— After this brief gen- eral discussion of the easily observable features of the nervous system as a whole, we are prepared to examine the system of NEURO-PHYSIOLOGICAL BASIS OF ACTION 137 pathways connecting the sense organs and the muscles. The first feature to study is how a spinal nerve is made up. We have already noted that there are thirty-one pairs of spinal nerves. Each nerve is made up of two roots, an efferent or motor root and an afferent or sensory root. The dorsal or afferent root (Fig.
FIG. 36. — Segment of spinal cord showing origin of a nerve. 1, afferent or sensory root of nerve; 4, efferent root of nerve; 3, spinal ganglion; this gives rise to the axones which form the afferent root; 2, a spinal nerve; it is made up of motor fibers and afferent fibers (for simplicity the various subdivisions of the nerve such as anterior, posterior and recurrent are left out).
36, 1) plunges horizontally into the cord, roughly at the tip of the dorsal horn. The ventral root (Fig. 36, 4) enters (really emerges) at the same level.
Upon each of the thirty-one afferent roots there is situated a swelling called the ganglion (Fig. 36, 3). This is a structure 138 PSYCHOLOGY easily observable without the aid of a microscope. It contains the cell bodies which give rise to the afferent peripheral processes and roots. A complete afferent neurone is shown on page 116. Attention was called there to the fact that these neurones have no dendrites unless we call the processes running out to the sensory surface the dendrite. The dorsal root (Fig. 36, 1) is made up of the typical axone process from these cell bodies. In man there are nearly one and a third million of axones in the dorsal roots of the two sides of the spinal cord, but less than one-half million in the ventral root. If the ventral root is fol- lowed back into the cord it will be found that it is really an out- growth from cell bodies lying in the ventral column (of gray matter). The genetic relations then are: cells in the ventral horn give rise to axones which leave the cord at a given horizontal level, the efferent roots. Each cell in the spinal ganglion at the same horizontal level gives rise to a single afferent axone which branches like a T or a Y, one process entering the cord as a dorsal root fibre, the other process (dendrite) going to a sense organ in the skin, muscle, tendon or joint. Just 'peripheral- ward to the spinal ganglion the motor fibers join the sensory. The combination is from then on called a nerve (Fig. 36, 2), for ex- ample, the first thoracic nerve on the right side. The nerve as a whole runs in a dense sheath of connective tissue, the epineureum (not shown). Somewhere in its course most of the afferent axones leave the epineural sheath to end in receptors (of muscles, ten- dons, joints and skin). The efferent axones enter the muscles and end in a typical way, shown in Fig. 47, page 161. Theoreti- cally it should be possible to destroy all of the afferent supply of neurones to a given skin and muscular area without destroying the efferent neurones. According to Head, some afferent neu- rones, however, always run with the motor fibers so that in order completely to rob an area of its afferent supply, both motor and sensory nerves would have to be cut.6 8 Head and Rivers have attempted to divide the cutaneous fibers up into a protopathic group ("including cutaneous pain, a diffuse non-localizable tactile sensibility, and the discrimination of extreme degrees of tempera- ture") and an epiciitic group ("light touch, cutaneous localisation, discrim- NEURO-PHYSIOLOGICAL BASIS OF ACTION 139 The Peripheral Neurones of the Brain. — There are twelve pairs of peripheral cranial nerves. Their superficial points of origin (efferent) or entrance (afferent) are shown on the ventral view of the brain (Figs. 30 and 31). The cranial nerves do not correspond at all closely to the pattern just described for the spinal nerves; some are wholly afferent, some wholly efferent, while others are mixed. The following table from Hardesty (Morris, Human Anatomy) gives the names and numbers of the nerves, whether they are afferent, efferent or mixed, and their distribution.
Name Nature General Distribution Olfactory (I) Sensory Olfactory region, nasal epi- thelium. Optic ( II ) Sensory Retina.
( Somatic.Eye-moving muscles. Oculomotor (III) Motor | yisceral ^.^ ^.^ Trochlear ( IV ) Motor-Somatic...Eye- moving muscles.
Abducens (VI) Motor-Somatic...Eye-moving muscles.
Trigeniinus; (V) Sensory Face, mouth and scalp.
Masticator (minor part or motor root of Trigeminus) Motor-Somatic...Muscles of mastication.
Somatic.Facial muscles.
Visceral.Salivary glands, vessels (?).
Glosso-palatine (intermedi- ( Sensory Tongue and palate.
ate part of facial) | Motor- Visceral..Salivary glands.
Cochlear ( auditory ) (VIII).Sensory Internal ear.
Vestibular ( equilibrator) (VIII ) Sensory Semicircular canals, utrt culus, sacculus.
f Sensory Tongue, palate, pharynx.
Gloasopharyngeal ( IX ).. J f Somatic Pharynx.
( Motor | Visceral.Glands and vessels.
( Sensory Alimentary canal,lungs,heart Vagus (X) -j T^otor ( Somatic, Larynx, pharynx.
larynx, trachea, lung.
ination of intermediate degrees of temperature, and some others"). The quotations/ are from Herrick. This work has been a great misfortune to neurology. It was accepted uncritically and on the basis of it the somatic conduction paths in the central nervous system have been delimited without sufficient confirmation. The works of Trotter and Da vies and of Boring have cast doubt upon the wholesale generalizations made by Rivers and Head.
( So Facial (VII) Motor.j y.
140 PSYCHOLOGY Ilypoglossal (XII) Motor-Somatic...ToBgue-moving muscles.
( Somatic: Neck and shoulder muscles. Spinal Accessory (XI)...Motor j Visceml.PiiarynXj larynx and heart.
Each of the afferent nerves or the afferent portion of the mixed nerves has a ganglion corresponding to a spinal ganglion. The ganglion, however, may be at some distance from the point of entrance into the central system: the ganglion for the cochlear division of the VIII nerve lies in the modiolus of the cochlea (ganglion spirale); for the vestibular division it lies blended within the nerve at the bottom of the internal auditory meatus (ganglion of Scarpa) (Fig. 8, 3). In the I or olfactory nerve it lies in the mucous membrane of the nose. In the II or optic the relations are peculiar; the first neurone, cell body and axone lie in the retina (Fig. 17). The cranial nerve marked II (optic) is not a peripheral nerve but a central tract.7 The V or trigeminus springs from cells in the semilunar (Gasserian) ganglion — which lies in Meckel's cave, a cleft in the dura mater in the upper surface of the petrous portion of the temporal bone. The sensory fibers of the facial, VII (Glosso- palatine), spring from cells in the geniculate ganglion. This ganglion is situated within the canalis facialis (Fallopii). The superior ganglion and the inferior ganglion of the afferent part of the IX (Glossopharyngeal) lie in the jugular foramen. The X has two ganglia — jugular, which lies also in the jugular fora- men, and the ganglion nodosum, which lies below the base of the skull and in front of the jugular vein.
A discussion of the peripheral distribution and the central connections of these nerves other than those of the special senses is a task too vast to undertake outside of an anatomical labo- ratory. We shall, however, indicate some of the more important central pathways of the afferent nerves in our discussion of the ascending and descending pathways in the cord and brain.
Course of the Ascending Neurones in the Cord and Brain.
Turning to the study of the further course of the afferent roots inside the central nervous system, we find in the spinal cord that they may establish several connections upon their entrance. The afferent root may end at that level or above or below it. After entrance it usually divides by a Y division (Fig. 23) sending The reason for this depends upon the embryology of the eye. The retina with its optic stalk (becoming later the optic tract) was originally a part of the embryological brain.
NEURO-PHYSIOLOGICAL BASIS OF ACTION 141 one branch downward (caudal) in the dorsal funiculus and the other branch upward in the dorsal funiculus (sometimes as far as the medulla oblongata, where it ends in a nucleus of recep- tion). At various levels each of these branches gives off collat- erals which end around the dendrites of cell bodies within the cord. The following relationships are known to occur: (1) such a collateral may end around the dendrites of a motor cell in the ventral horn on the same or on the opposite side, forming the dorsal root dorsal root FIG. 37. — Diagram of some of the types of connections between the afferent fibers of the dorsal root and the motor fibers of the ventral root (rabbit). The various fibers are numbered: 1 and la represent collateral branches of the dorsal root fibers which run directly to the dendrites of the motor neurones in the ventral column of the same or opposite side; 2, dendrites of ventral column cells crossing to the opposite side, meeting with the ter- minals of 1 above (probably a very rare connection); 3a, 3b and 3c are central neurones; these neurones may connect the sensory root ending with the motor neurone in the same segment on the same or opposite side or they may send their axones to end at lower or higher levels; 4 is a collateral of the motor axone returning to end in the gray matter around other cells. (Herrick's Introduction to Neurology.) W. B. Saunders Co.
direct reflex pathway or the pathway for the crossed reflex, (2) the dendrites of the ventral horn cells themselves may cross to the opposite side in the central commissure and end in contact with a collateral of an afferent root. In either 1 or 2 above a short central interconnecting neurone may be interpolated. Its cell body lies in the dorsal horn and its axone ends around ventral cells of that or higher or lower levels. Certain other relationships which may occur will appear when we come to consider special tracts in the cord. Fig. 37 shows some of the possible connections.
PSYCHOLOGY PSYCHOLOGY Connection of Cord with Cerebellum (Ascending Neu- rones.)— Certain of the collaterals and axones of afferent roots end around cells in Clark's column (Fig. 38, 19), which is to be found in the posterior horn. Two tracts arise from these cells. They ascend in the lateral funiculus. One of the tracts (tractus 12. A3 spino-cerebeilaris dorsalis, also called di- rect cerebellar tract and Flechsig's tract) (Fig. 39, 7), enters the cerebellum via the inferior peduncle, the other (tractus spino- cerebeilaris ventralis) (Fig. 39, 5) enters the cerebellum by way of the superior ped- uncle. The two tracts carry impulses from the kinaesthetic sense organs to the cere- bellum. We shall see later that the cerebel- lum is an important central station for all impulses connected with the maintenance of equilibrium and the tone of the muscles.
cord.' 14, ventral median fis- r«^» ^* r /-« j «^i -HIT i 11 sure (fissuramediana superior); Connection OI Cord With Medulla 13, dorsal median septum / j /~\ \A i« -».T (sulcus mediana posterior); (and Cortex), ASCCndmg NeUFOnCS.
missura posterior alba); ii.ven- -1^6 afferent libers coming from the musanterior alba); 21, entrance ClCS, tendOUS and JOintS (kmSBSthetlC SCUSe of dorsal root (radix posterior): N,, IT,-, 20, cell bodies lying in the organs) enter the cord and take up a (nucleus dorsalis); is, cell vertical position in the dorsal funiculi bodies lying in lateral column; /,,.
these; give rise to the pregan- VpOSteriOr COlUmnS) and aSCCnd Without glionic fibers, axones, which run •.,.,,.
to thesympathetic ganglia: i?, interruption (that is, without synaptical 16, 15, groups of cell bdoies,.
giving rise to the peripheral Connections) as far as the medulla motor neurone (radix anterior). -, -,. n oblongata (first relay station). They end there around definite cell groups called nuclei of reception. The cells! are so numerous that their presence is marked by an actual swelling shown in Fig. 32, 23 and 4. There are really two reception nuclei in each dorsal funiculus, the nucleus funiculi gracilis and the nucleus funiculi cuneati (the dorsal funiculus as a whole being divided into two fasciculi, the gracilis lying nearest the median septum and the cuneatus occupying the remainder of the space (Fig. 39, 10 and 9). The cell bodies lying in these nuclei give off axones (ascending neurones of the second order) which swing over to the opposite side, passing under the spinal NEURO-PHYSIOLOGICAL BASIS OF ACTION 143 canal, which in this region is near the dorsal surface (Fig. 40). This crossing is known as the sensory decussation (decussatio lemniscorum). The fibers after crossing form the medial lem- niscus (lemniscus medialis), which can be seen in every cross- section from the medulla to the thalamus. The lemniscus is being constantly enlarged because ascending neurones of the sec- FIG. 39. — Diagram through spinal cord in mid-cervical region to show arrangement of various fiber tracts. 1, ventral median fissure; 8, dorsal median septum; 9, 10, show the two subdivisions of the dorsal funiculus, 9 being the fasciculus gracilis, 10, the fasci- culus cuneatus; 2, one of the ascending tracts to the thalamus, tractus spino-thalamicus ven- tralis; 3, tract connecting spinal cord with the olives, tractus spino-olivaris; 4, tract connecting cord with the roof of the mid-brain, tractus spino-tectalis; 5, tractus epino- cerebellaris ventralis (part of the fasciculus antero-lateralis superficialis); 6, tractus spino- thalamicus lateralis; 7, tractus spino-cerebellaris dorsalis (fasciculus cerebello-spinalis).
ond order from the cranial afferent nerves (after decussating) are constantly joining. These ascending neurones in the medial lemniscus end around the cell bodies in the thalamus on the same side (second relay station). The cells there give rise to axones (ascending neurones of the third order) which pass through the internal capsule (Fig. 44, 20) through the corona radiata (Fig. 44, 16) then they ray out to end in the gyri behind the central sulcus — the so-called somaesthetic reception area in the cortex. Summarizing: In this system (kincesthetic) the peripheral end of the first afferent axone terminates in a sensory structure in 144 PSYCHOLOGY muscle, tendon or joint. The central end^nters the cord, turns upward and reaches the medulla (ascending neurone of the first order). The neurones of the second order start in the medulla and end in the thalamus (of the opposite side). The neurones of the third order (the final neurones) start in the thalamus and end in the cortex.
Connection of Cord with Thalamus (and Cortex), Ascend- ing Neurones. — Certain of the afferent neurones (spinal roots), instead of ascending in the dorsal funiculus (dorsal column), end in the posterior horn. The cell body of a central neurone lying there sends an axone across the cord (via the ventral commis- sure) to the ventral and lateral funiculi of the opposite side, forming the two spino-thalamic tracts (Fig. 39, 2 and 6). The axones in these tracts carry impulses from the temperature, pain and pressure senses. The fibers run directly up to their nuclei of reception in the thalamus (ventral and lateral nuclei of the thal- amus). In other words, the temperature, pain, pressure pathway is relayed once, immediately upon entrance into the cord, and not again until the thalamus is reached (Fig. 40).
Above the level of the medulla they join the medial lemniscus and follow the same route as the axones just described, terminat- ing probably in adjacent cortical regions (somaesthetic area).
Ascending Pathway of Afferent Cranial Neurones. — The afferent root of the V nerve (trigeminus) has its nucleus of reception (first relay station) in the gray matter of the pons. The reception nucleus is large at this level but tapers down, end- ing at the upper level of the spinal cord. The cells in this nucleus send axones which, after crossing, probably join the medial lemniscus (ascending neurones of the second order) and end in the thalamus (Fig. 40). There a second relay occurs and the axones from the thalamic cells (ascending neurones of the third order) pass up through the internal capsule and corona radiata and end in the soimesthetic area. It is probable that this ascend- ing tract of the V remains fairly distinct from the fibers of the lemniscus (in Fig. 40 it is marked trigeminal lemniscus). The glossopalatine (afferent portion of VII, n. intermedius) entering under the inferior border of the pons (Fig. 31, 24), has its nucleus NEURO-PHYSIOLOGICAL BASIS OF ACTION 145 of reception in the gray matter of the pons. Cells situated there send axones into the medial lemniscus to end in the thalamus. Neurones beginning there continue the pathway to the somaes- thetic area. The axones here spoken of carry kinsesthetic and cutaneous impulses from the tongue and palate. The central cerebral __cortex tngemmal lemniscus s Kin funiculus ventral pyramidal I' ', I +1 — lateral pyramidal Tract - spinal 5anfllion sKin muscle FIG. 40. — Showing some connections between the cord and brain. Explanation of figure in text. (Herrick's Introduction to Neurology.)
pathway of the glossopalatine axones carrying taste impulses from the anterior two-thirds of the tongue is unknown. They may possibly run to the thalamus and after being relayed there enter the gyrus hippocampus in the basal part of the temporal lobe (Fig. 30).
The glossopharyngeal (afferent portion of the IX) and the 10 146 PSYCHOLOGY afferent division of the vagus (X) enter%the medulla lateral to the olive and end in the nucleus tractus solitarii and neighbor- ing cells in the medulla. These cells send axones to join the lemniscus of the opposite side: they also bear cutaneous, kin- a?sthetic and organic impulses. The course of the gustatory ax- ones through the brain stem (the glossopharyngeal innervates the posterior third of the tongue) and higher structures is likewise unknown.
Course of the Nerves of Special Sense and Their Cortical Terminations. — The auditory nerve (cochlear division) enters the medulla lateral to the olive (Fig. 31, 25) and ends around two reception nuclei in the tuberculum acusticum (Fig. 32). The cells from these nuclei send axones across the mid-line (stria medullaris acustica, Fig. 32, 6) and ascend along the sides of the medial lemniscus, forming the lateral lemriscus (lemniscus lat- eralis). These ascending neurones of the second order end mainly in the medial geniculate body of the thalamus (Fig. 32, 10). Some end undoubtedly in the inferior colliculus (Fig. 32, 18). By a short system of neurones the pathway of the latter group is con- tinued to the medial geniculate body. The third (or, as the case may be, the fourth) order of neurones carries them from this point through the internal capsule (Fig. 44, 20) to the cortical recep- tion station in the temporal lobe (gyri immediately below the fissure of Sylvius) (Fig. 35, 4).
The fibers from the vestibular portion of the eighth nerve enter the medulla at a slightly higher level than the cochlear branch (Fig. 31, 25). The fibers end around four terminal nuclei in the floor and wall of the fourth ventricle: Deiter's (nucleus nervi vestibuli lateralis) situated just internal to the restiform body; a superior, Von Bechterew's (nucleus nervi vestibuli su- perior) situated dorsal to Deiter's nucleus in the lateral wall of the fourth ventricle; a medial, Schwalbe's (nucleus nervi ves- tibuli medialis); and an inferior (nucleus nervi vestibuli spi- nalis). Axones from the lateral and inferior nuclei form paths to the spinal cord. Axones from the lateral and superior nuclei form a path to the roof nucleus of the cerebellum (nucleus fas- tigii) of the opposite side. Apparently there is no thalamic tract NEURO-PHYSIOLOGICAL BASIS OF ACTION 147 issuing from these nuclei. It would seem that most of the con- nections of the vestibular portion of the ear are thus made with the cerebellum and cord.
As was pointed out, the peripheral optic nerve lies in the retina (neurones of the first order). The layer of ganglion cells with their fibers are ascending neurones of the second order cor- responding to the tract running from cerebellum to thalamus in the kinaesthetic system. These neurones are rightly named the optic tract (tractus opticus) (Fig. 31, 9). The axone system in part crosses at the chiasma (Fig. 31, 14). From the chiasma the fibers enter the brain in the region of the pulvinar of the thal- amus. Reaching the pulvinar, the optic tract divides into a Olfactory tract \Granule cell Mitral cell Glomerulus 'Olfactory nerve •Ethmoid bone ^*"*^ Olfactory epithelium FIG. 41.— Diagram of the connections of the olfactory bulb. (Herrick's Introduction to Neurology.)