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of the lateral occipital lobe, particularly if the lesion is a large one, in the left hemi-
sphere, or if lesions of both occipital lobes are present. A lesion of the cuneo-
calcarine cortex causes lateral homonymous hemianopsia. This may be produced



12 14 Hr.MAX ANATOMY.

also by a lesion in the lateral portion of the occipital lobe, if it extends inwards
sufficiently to interrupt the optic radiations.

In spite of extensi\ e researches the functions of the central ganglia are very
little known.

Lesions of the cerebellar hemispheres may not produce distinct phenomena
until the median lobe or vermiform process is involved, when two especially charac-
teri-stic svmjjtoms will almost certainly develop. These are a peculiar disturbance of
equilibrium with a staggerinj:>- gait (cerebellar ataxia), and a troublesome vertigo.
Although the patient can scarcely stand alone he may possibly be able to perform
the most delicate movements with his upper extremities. The vertigo occurs only
in standing or walking, and is then almost always present. Nystagmus is also a
frequent symptom. X'omiting is very often present, but is not characteristic, since
it is equally frequent in other brain diseases.

Extending along the floor of the aqueduct of Sylvius and of the fourth \ entricle,
that is, along the cerebral peduncles, pons and medulla, we find the nuclei of origin
of the motor fibres of the cranial nerves. It should be borne in mind that the con-
trolling centres of these nerves are in the cerebral cortex. Many automatic centres,
as of circulation, respiration, sweating, and regulation of heat, as well as the motor
and sensory tracts are found in the medulla.

Cranio-Cerebral Topography. — In order that the surgeon may expose and
recognize certain areas of the cortex, it becomes very important that the relations
between these areas and the corresponding external surface be well understood. For
this purpose advantage is taken of the landmarks of the skull (page 241). From
these bony points, ridges and depressions, by means of lines and measurements, the
known cortical areas may be accurately mapped out.

The iippcr limit of each cerebral hemisphere is indicated, approximately, by the
median line at the top of the skull from the glabella to the external occipital protu-
berance, due allowance being made for the superior longitudinal sinus, which lies
under the skull, in the longitudinal fissure, between the two hemispheres.

The lower limit is represented by a transverse line, in front, just above the upper
margin of the orbit. At the side of the skull the line passes from about a half inch
above the external angular process of the frontal bone to just above the external
auditory meatus. From here it passes to the external occipital protuberance ; this
part of the line corresponding, approximately, to the lateral sinus. The cerebellum
lies immediately below this line.

Of the brain fissures, those of greatest importance in cerebral localization are
the Rolandic and Sylvian, since by means of these all the best known cortical centres
can be located. Of the two, the fissure of Rolando is much the more important,
because the motor, the most definitely known cortical area, is associated with it. Its
upper limit is at a point about 12 mm. Cone-half inch) behind the mid-point between
the glabella and the inion, arid about one-half inch from the median line. It passes
outward, downward, and forward, approximately, at an angle of 71° with the median
sagittal line of the skull. It is 8.5 cm. (333 in.) long (Thane), and ends below just
above the fissure of Sylvius. Near its lower end it turns rather suddenly downward,
so that, in this part, it is not in the line of the angle of 71°.

Many methods have been devised for the purpose of making the line of the
fissure on the scalp.

Chietie' s method consists oi folding an ordinan*' square sheet of paper on the
diagonal line, thus dividing an angle of 90° in half, making two of 45°. One of these
angles of 45° is again halved in a similar manner, making two new angles each of
2254°. The paper is then so unfolded that one of the angles of 22^4 ° is added to that
of 45"^, making a new angle of 673^° ; this will be sufficiently near that of the fissure
of Rolando for all practical purposes.

Horsley s cyrtometer consists of two strips, either of thin, flexible meta) or of
parchment paper, each graduated in inches. The lateral arm is placed at an angle
of 67° with the long arm, the apex of the angle being at a point 12 mm. or one-half
inch behind the mid-point of the long arm.

Le Fort simply drew a line from the beginning of the fissure, above, to the mid-
dle of the zygoma, below, and marked off on this line the proper length of the fissure.



rRACllCAI. C()N.Sini:RATlUNS : TUl-: HRAIN.



1-^15



Anderson ;uul Mackins sumni-st : ( i ^ ;i im-dian saj^iltal liner lri)in the glabella lo
'thcinion ; (2j a frontal line from the inicl-sa^ittal point to the depression jnsi in front
of the ear at the level of the upper border of the meatus ; (3; a squamosal line fn^m
the most external point of the txternal anj;ular process, at the level »jf the superior
border of the orbit to the junction of the middle ami lower thirds of the frontal line,
and prolonj^ed for about 3.7 cm. ( 1 3^ in. j behind the frontal line. The uj)|jer ex-
tremity of the central fissure was found by them to lie between the mid-sagittal jjoint
and a point iS mm. ( -'^ in. ) behind it, and the lower extremity of this fissure they
located near the scpiamosal line, about 18 mm. (^ in. ) in front of its iinuti«)n with
the frontal line. The connnenct'ment of the lateral portion of the .Sylvian fissure is
not at a definite fixed point, but will usually be hit at a point from 3.7-5 cm. ( lyd-'Z
in.) behind the angular process, the course of the horizontal |;orti<jn of this
fissure corresponding closely to the s^iuamosal line (Mills).



Fissure of Kolaii



Line for Kol.-mdic fis.sure



Inlerp.Trietal fissti



External parieto-
occipital fissure



Parietal eminence




Posterior limb
Sylvian fissure



ne for
Sylvian fissure



Vertical limb

of Sylvian fissure

Horizontal limb
of Sylvian fissure
Glabella

Nasion



Lateral sinut



Semidiagrammatic view of head, showing relation of Rolatidic antl Sylvian fissures and lines.

The fissure of Sylvius begins anteriorly, appro.ximately, at a point 3 cm. (i^^
in. ) behind the external angular process of the frontal bone ; and ends posteriorly at
a point 18 mm. (3/^ in.) below the parietal eminence. A straight line between these
two points will represent the fissure, which is about 10 cm. (4 in. ) long. The an-
terior 18 mm. (^ in.) of this line will correspond to the main portion of the fissure
and the remainder to the horizontal limb. The vertical limb ascends for about
2.5 cm. (i in.) from the posterior end of the main fissure. Around the posterior
end of the horizontal limb, and approximately under the parietal eminence lies the
supramarginal convolution. It is continuous in front with the ascending parietal
convolution, and behind with the angular gyrus.

The parieto-occipital fissure is most marked on the mesial surface of the brain.
The e.xternal limb passes outwards, almost at right angles to the longitudinal fissure
on the external surface for about 2.5 cm. and lies from 2-3 mm. in front of the lambda.

Th^frofital lobe is divided into three main convolutions by the superior and in-
ferior frontal sulci. The line for the superior frontal sulcus passes directly backward



I2l6



HUMAN ANATOMY.



from the supraorbital notch, and parallel to the longitudinal fissure to within i8
mm. ( ;^ in. ; of the fissure of Rolando. The inferior frontal sulcus is represented,
approximately, by the anterior end of the temporal ridge.

In the parietal lobe the most important sulcus is the intraparietal. It begins
near the horizontal limb of the fissure of Sylvius, and passes upward and backward
about midway between the fissure of Rolando and the parietal eminence. It then
turns backward, running about midway to the longitudinal fissure and the centre
of the parietal eminence. Above the sulcus, in front, lies the ascending parietal
convolution, just posterior to the fissure of Rolando and behind the superior pari-
etal lobule. Below the sulcus, anteriorly,, is the supramarginal convolution, and
posteriorly, the angular gyrus.

Fig. 1045.

Ere^ma



Lateral ventricle



Middle meningeal

rten-, anterior
branch



Posterior horn of
lateral ventricle



Lateral sinus



Middle meningeal artery, posterior
branch ; inferior horn 01 lateral
ventricle seen beneath




Semidiag^rammatic view of head, showing position of ventricles, lateral sinus and middle meningeal arteries

as projected on skull.



The temporal lobe lies below the fissure of Sylvius and e.xtends forward as far as
the edge of the malar bone. • The first temporal sulcus lies about one inch below and
parallel with the fissure of Sylvius, and the second about i8 mm. (3^! in. J lower.

The occipital lobe lies posterior to the parieto-occipital fissure and the tem-
poral lobe.

The motor tracts are made up of the fibres passing from the motor portion of
the corte.x in the Rolandic region to the motor nuclei from which arise the nerves
supplying the muscles which the cortical areas control. After leaving the cortex the
fibres pass downward in the corona radiata, and converge to the posterior limb of the
internal capsule. The motor fibres of the cortico-bulbar and cortico-spinal tracts,
occupy the genu and adjacent third of the internal capsule (page 1188), although
Dejerine holds that the whole posterior limb is motor. They continue their course
downward through the crura cerebri, pons, and medulla ; in the lower part of the
latter the greater number cross to the opposite side and pass down in the cord as the
lateral or crossed pyramidal tract. A small number, sometimes absent, pass down



I'RACriCAI. CONSIDIIKATIONS : Till-: HRAIN. 1217

on thf siinu- side. \Vc have already seen that k-sions of the c(jrtex produce inono-
pletjia, unless lar^e enough to involve the whole niot(»r zone, but ((^rtical hemiplegia
is much more common than cortical monoplegia. In the internal capsule the motor
fibres are i^athereil toijether so compactly that a small lesicjn, as an ajjoplectic hemor-
rhage, will fre(|uently interrupt the whole tract and give a hcniiplegia of the oj)posite
side of the biulv.

In the metluUa and cord tlu- tracts of'both sides are so close together that a
lesion may easily paralyze both sides ( j)araplegia) ; indeed, diseases of the cord fre-
quentlv in\()lve the whole transverse section, paralyzing sensatif)n as well as motion.

" Regarding disturbances of sensation, it is of importance to recall the anatomical
relations of the chief sensory paths, the mesial fillet and the sjjino-thalamic tract.
The former arises from cells within the gracile and cuneate nuclei of the medulla,
arouiul which cells the long fibres of the posterior column end. The fibres of the
fillet-tract, therefore, cross in the sensory decussation within the medulla. The fibres
of the spino-thalamic tract, on the other hand, are a.xones of spinal cells situated on
the opposite side and undergo crossing within the cord. Within the brain-stem, the
two paths are closely associated and lesions within the medulla may involve both sets
of fibres, leading to comjilete hemi-ancsthesia of the ojiposite side. Unilateral lesions
of the cord, on the contrary, produce only partial hemi-anesthesia, since within the
cord the tracts ascend on different sides. ' '

Hcmipinria is, therefore, the common form of cerebral paralysis ; paraplegia the
common form of spinal paralysis ; while monoplci^ia occasionally results from lesions
of the brain corte.x, but more commonly from lesions of peripheral nerves.

The sides and convexity of the brain can be exposed for operation, so that lesions
of the cortex can be attacked and often removed ; but the region of the internal
capsule, which is near the basal ganglia, cannot be reached.

The soft brain may be injured by contact with its bony walls when the head is
violently shaken, the spaces surrounding the brain and filled with fluid permitting
considerable moxement of the brain. The injury in cerebral contusion occurs more
frequently on the under surface, both as regards the cerebrum and cerebellum, than
on any other part ( Prescott Hewett). That portion, however, which includes the
medulla, pons, and interpeduncular space, rests on a large collection of cerebro-
spinal fluid, and is least frequently injured.



THE PERIPHERAL NERVOUS SYSTEM.



In a broad sense and as contrasted with the cerebro-spinal axis, the peripheral
nervous system includes all the nerve-paths by which the various parts of the body
are brought into relation with the brain and spinal cord. These j^aths embrace, in
a general way, two groups. One group, the somatic nerves, includes the nerves

Fig. 1046.



Olfactorj' bult)



Orbital surface
of frontal lobe



Temporal lobe

Anterior
fterforated space

Mammillary
bodies

Cerebral
peduncle

Pon



Medulla,
pj-raniid

Hypoglossal
ner\-e



Cerebellum




Olfactorj- tract
Optic ner\-e, cut



Optic
commissure



Optic tract



Oculomotor
ner\'e

Trochlear nerve
Trigeminal
ners-e

Abducent nerve

Facial ner\-e

Auditory nerve
Glosso-pharyn-
geal ner\-e
Pneumogastric
nerve
Spinal

accessory- ner\-e
spinal portion

Pyramidal
decussation



Spinal part of
XI. ne^^•e



Occipital lobe



Anterior roots of spinal nerves



Inferior aspect of brain, denuded of its membranes, showing superficial origins of cranial nerves ; origin of trochleai
nerve is on dorsal surface and therefore not seen.

supplying the voluntary muscles, integument and organs of special sense ; the sec-
ond group, the visceral nerves, includes those supplying the involuntary muscle
throughout the bodv and the thoracic and abdominal viscera. The somatic nerves
are subdi\-ided into (a) the cranial nerves, which are attached to the brain and pass
through foramina in the skull, and (<5) the spina/ nerves, which are attached to the
spinal cord and traverse the intervertebral foramina. The visceral, or splanchnic
121S



THE CRANIAL NERVES. 1219

nerves, although directly or indirectly connected with the cerebro-s|>inal axis, pre-
sent peculiarities aiul, as the system of sytnpalhctic nerves, are accorded, at least for
convenience (jf description, a certain degree of independence. While by no means
all of the spinal nerves contribute sj)lanchnic branches — such branches being given
off especially by the thoracic and upper lumbar nerves — they all receive sympathetic
filaments, which form, therefore, integral parts of the somatic nerves. From the
sympatlulic neurones of the ganglialed cords axones pass, by way of the gray rami
communicantes (page 1357), to the trunks of the spinal nerves and thence by these
are carried to all parts of the body for the supjily of the involuntary muscle occur-
ring within the blood-vessels and the integument and for the cutaneous glands. Fur-
thermore, it must be remembered, that although the predominating constituents of
a spinal nerve may be axones derived from anterior horn root-cells and destined for
voluntary muscle, such trunk also contains a number of afferent fibres which convey
impulses received from the neuromuscular and neurotendinous sensory endings, the
nerve-trunks reckoned as "motor" in all cases, when analyzed, being found to con-
tain sensory and sympathetic fibres as well as efferent ones.

THE CRANIAL NERVES.

The cranial nerves (ncrvi cerebralcs) include twelve pairs of symmetrically
arranged nerve-trunks, which are attached to the brain and, traced peripherally,
escape from the skull by passing through various foramina at its base to be distrib-
uted for the most part to the structures of the head.

The point at which a cranial nerve is attached to the surface of the brain is
designated its superficial origin ; the group of more or less deeply situated nerve-
cells with which its fibres arc directly related is often spoken of as its deep origin.
From what has been said (page 1278) concerning the position of the cell-bodies of
motor and sensory neurones, it is evident that only the motor fibres of the cranial
nerves spring from nerve-cells within the cerebro- spinal axis, while the fibres con-
ducting sensory impulses arise from nerve-cells situated within ganglia lying outside
the central nervous axis and somewhere along the course of the nerve-trunks. It
follows, therefore, that the term "deep origin," as applied to the cell-groups within
the brain, can properly relate only to the origin of motor fibres ; the cell-groups with
which the sensory fibres come into relation after entering the brain-substance are in
reality nuclei of reception, or of termination, and not of origin. The sensory
impulses so received are transmitted to various parts of the brain by the more or less
complex paths afforded by the aeurones of the second, third, or even higher order.
In addition to their relation to the deep nuclei, whether of origin or of reception, the
fibres of every cerebro-spinal nerve are directly or indirectly influenced by neurones
situated within the shell of gray matter that covers the cerebrum. The position of
these higher cortical centers, as they arc termed, is known with considerable
accuracy for many groups of nerves, but regarding "Others more definite data con-
cerning cerebral localization must be awaited.

Bearing in mind the foregoing distinctions, for convenience we may follow the
conventional description in which all the nerves are regarded as passing away from
the brain, the direction in which they convey impulses, centripetally or centrifugally,
being for the time disregarded.

On leaving the surface of the brain at its superficial origin, each cranial nerve,
invested by a sheath of pia mater, traverses for a longer or shorter distance the sub-
arachnoid space, pierces the arachnoid and from the latter acquires an additional,
but usually not extensive, sheath. It then enters a canal in the dura mater that
leads to the foramen in the skull, through which the nerve escapes from the cranium,
invested by a sheath prolonged from the dura which is continuous with the epi-
neurium covering the nerve-trunk. The position of the dural aperture and that of the
foramen bv no means alwavs correspond, some of the nerves, notably the fourth and
sixth, pursuing an intradural course of some length before gaining their osseous exit.

According to the order in which they pass through the dura lining the cranium,
the pairs of cranial nerves are designated numerically from the first to the twelfth.
They are further distinguished by names based upon their distribution or functions.



I220



HUMAN ANATOMY.



Number.


Name.


I.


Olfactory :


•II.


Optic ;


III.


Oculomotor


IV.


Trochlear :


V.


Trigeminal :


VI.


Abducent :


VII.


Facial :



Certain of the cranial nerves are entirely motor ; some convey the impulses of special
sense ; while others transmit impulses of both common sensation and motion. A
general comparison of these relations, as now usually accepted, is afforded l)y the
following summary :

THE CRANIAL NERVE.S.

Function.

Special .sense of smell.

Special .sense of sight.

RIotor to eye-muscles and levator pal-
pebral superioris.

Motor to superior oblique muscle.

Common sensation to structures ol head.

Moior to muscles of mastication.

Motor to external rectus muscle.

Motor to muscles of head (scalp and
face) and neck (platysma).

Probably secretory to subma.xillary and
sublingual glands.

Sensory (taste) to anterior two-thirds of
tongue.

Hearing.

Equilibration.

Special sense of taste.

Common sensation to part of tongue
and to pharyn.x and middle ear.

Motor to some muscles of pharyn.x.

Common sensation to part of tongue,
pharyn.x, ccsophagus, stomach and
respiratory organs.

Motor (in conjunction with bulbar part
of spinal accessory) to muscles of
pharynx, cesophagus, stomach and
intestine, and respiratory organs ;
inhibitory impulses to heart.

Spinal Part : Motor to sterno-mastoid
and trapezius muscles.

Motor to muscles of tongue.



VIII.
IX.



Auditory,

{a) Cochlear division :
{d) Vestibular division ;

Glosso-Pharyngeal :



Pneumogastric or Vacjus;



XI.

XII.



Spinal Accessory :
Hypoglossal :



Practical Considerations. — Lesions may affect a cranial ner\e within the
brain or in its peripheral portion. A central lesion clinically is one above the nucleus
of the nerve, and may be cortical or may encroach upon its intracerebral connections.
It may merely irritate the nerve or may paralyze it. By a peripheral lesion is meant
one involving the nucleus or th^^ fibres of the nerve below the nucleus.



THE OLFACTORY NERVE.

The olfactory nerve (n. olfactorius), the first in the series of cranial nerves,
presents some confusion in consequence of the name, as formerly employed, being
applied to the olfactory bulb and tract as well as to the olfactory filaments — struc-
tures of widely diverse morphological values. As already pointed out (page 1151),
the olfactory bulb and tract (Fig. 993), with its roots, represent, as rudimentary-
structures, the olfactory lobe possessed by animals in which the sense of smell is
highly developed. It is evident that these structures, formerly regarded as parts of
the first cranial nerve, are not morphological equivalents of simple paths of conduc-
tion. On the other hand such paths are represented by a series of minute filaments,
the true olfactory nerves, that connect the perceptive elements within the nasal
mucous membrane with the rudimentary olfactory lobe.

The olfactory nerves proper, some twenty in number, are the axones of the
peripherally situated neurones, the olfactory cells (page 1414). which lie within the
limited olfactory area. The latter embraces in extent on the outer nasal wall chiefly



THE OLFACTORY NRRVH.



I22I



the mesial surface of the superior turbiiiale hour and a somewhat larger held on
the adjacent upper part of the nasal septum. The olfactory nerves (Fig. 1048),

Kk;. 1047.



Olfactory bulb

><Jl factory ncrvc-fibrcs

An upper ant. nasal br.
^-^leckels ganxliun

Nasal ntrve, ext. br. -/7*^ '.^^•"^^^L^^^ / ., Xp,„.r ,,osl. nasal brs.

/ Meckel's giinglion

^^^,_, _, ^ ^ ^ ^_^_^_„.^.m._ ^^^^^_.. ^ -'Naso-palatiue nerve
Exit ext. l>r. nasal nerve ' '" ~' "" " "" '""" ^



f Sup. ant. nas<'il br. of
Meckel's KanK'- and

I inf. ant. nasal br. of

\ ant. descending

I ]>alatine nerve
.\ jxjsterior nasal br.
Meckel's ganglion



\
Ant. descending palatine nerve, tlie middle palatine appearing posteriorly

Right na.sal fossa showing; distribution of olfactory and nasal nerves on lateral wall ; mucous nitnibrane has

been partly removed to expose nerves.

whose fibres are nonmedullated, exhibit a ple.xiforni arrangement within the deeper
part of the nasal mucous membrane, pass upward through the cribriform plate of




Fig. 1048.



Crista galli



Nasal nerve



Int. (septal) br. of nasal nerve
olfactory bulb



Ext. br. nasal nerve, cut



Naso-palati'ne nerve




Olfactory nerve-fibres
^Sphenoidal sinu-

Kx\ upper ant. nasal br. of
.Meckel's ganglion
Naso-palatine nerve

hx\ upper ant. nasal br
of Meckel's ganglion



l-'ustachian orifice



A'omer, posterior border



Soft palate, cut niesially



Right na.sal fos.sa showing distribution of olfactory and nasal nerves on septal wall; mucous membrane has

been partly removed to expose nerves.

the ethmoid bone and enter the under surface of the olfactory bulb. Within the
latter the nerve-fibres end in terminal arborizations in relation with the dendritic
processes of the mitral cells (Fig. 995), sharing in the production of the peculiar
olfactory glomeruli.



1222



HUMAN ANATOMY.



Central and Cortical Connections. — The impulses conveyed by the olfaetorj- nerves and
received by the mitral cells of the olfactory bulb, which cells may be rej^arded as constituting
the end-station or reception-nucleus of the peripheral path, are carried to neurones situated either
within the gray matter of the olfactory tract, the anterior perforated space or the adjacent part
of the .septum lucidum (Fig. 1049). Fibres connecting the olfactory centres of the two sides pro-
ceed from the cortex of the tract by way of the anterior commissure, forming \.\\ii pars olfactoria
of the latter, to end in relation with the cells within the opposite tract or bulb. From these



Online LibraryGeorge A. (George Arthur) PiersolHuman anatomy : including structure and development and practical considerations (Volume v.2) → online text (page 35 of 160)