Title: Dissection of the Platana and the Frog
Author: Bonde, C. von, Gilchrist, J. D. F.
Language: English
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UNIVERSITY OF CAPE TOWN.

Zoological Department.


DISSECTION OF

The Platana and the Frog


BY

J. D. F. GILCHRIST, M.A., D.SC., PH.D.,

_Professor of Zoology in the University of Cape Town_,

AND

C. VON BONDE, M.A.,

_Lecturer in Zoology in the University of Cape Town_.

[Illustration]

1919.

CAPE TIMES LIMITED, CAPE TOWN.



PREFACE AND INSTRUCTIONS.


Owing to the fact that some of the types of animals available for the
study of Zoology in South Africa differ from those described in text
books for students in other countries, it has been found necessary to
draw up special directions and diagrams for laboratory use in this
country. These are now put in the more convenient form of a text
book, which, though intended primarily for the use of the students of
the University of Cape Town, may, it is hoped, prove useful to others
engaged in the teaching and study of Zoology in South Africa.

The general plan of the course intended to be followed is first an
introductory study of a type of vertebrate. The Platana or Clawed
Toad, _Xenopus lævis_, is the most convenient type for this in South
Africa and in some respects is more instructive than the Frog.

Xenopus is therefore chiefly dealt with, but Rana can also be used,
the differences in this type being indicated in italics. It is
recommended that both types be studied where possible.

Only these two types are dealt with here, but may be followed by
descriptions of other South African types, such as the South African
Crawfish, Dogfish, Earthworm, etc.

It is intended that the student should make drawings of his
dissections on the blank page. He should not copy the diagrams, but
draw what he actually sees, and be prepared to demonstrate the parts
to the teacher.

The instruments required are a large and a small pair of scissors, a
large and a small scalpel, moderately fine forceps, a seeker, and
pins.

We are indebted to Mr. W. von Bonde, M.A., for the drawing of Figs.
3-5, 7-10.


  J. D. F. G.
  C. v. B.

  _Zoological Laboratory,
  University of Cape Town,
  June, 1919._



THE PLATANA (_Xenopus lævis_).

THE FROG (_Rana fuscigula_).


_First Day._


External Characters.

1. Make a drawing of the animal viewed from above, showing:--

    (_a_) The nostrils.

    (_b_) The eyes.

    (_c_) The nictitating membrane and a short tentacle beneath the eye.
    [_The tentacle is absent in the Frog._]

    (_d_) The tympanic membrane, which is not shown in the Platana, but
    is well marked in the Frog.

    (_e_) The opening of the cloaca.

    (_f_) The limbs and their divisions.

    (_g_) The skin with tube-like lines round the body. [_In the Frog
    these lines are not present._]

[Illustration: FIG. 1.--Xenopus lævis and Rana fuscigula.]

2. Make an enlarged drawing, about twice the natural size, showing:--

    (_a_) The fore-limb with the digits.

    (_b_) The hind-limb with the digits. The inner three digits have
    claws. [_The claws are absent in the Frog._]

Name the parts of these limbs, and number the digits from within
outwards, noting that the first is absent in the fore-limb. Mouth
Cavity.

Open the mouth cavity widely, and make a sketch showing:--

    (_a_) The teeth on the upper jaw.

    (_b_) There are no teeth on the vomer. [_In the Frog each vomer is
    supplied with teeth._]

    (_c_) The internal openings of the nostrils.

    (_d_) The single opening of the Eustachian tubes. [_In the Frog
    there is an opening on each side._]

    (_e_) There is no tongue. [_A tongue is present in the Frog._]

    (_f_) The glottis.

    (_g_) The gullet.

Owing to the complexity of the Eustachian tubes and the bony form of
the ear cavity, it is not possible to pass a seeker from the
Eustachian aperture through the tympanic membrane. [_In the Frog the
Eustachian tubes are short and straight and permit of this being
done._]

[Illustration: FIG. 2.--Mouth cavity of Xenopus and Rana.]


Muscular System and Vascular System (_Part_).

1. Pin the Platana on its back in water, and cut through the skin
with scissors along the middle line, throughout the whole length of
the body. Pin back the skin on each side, and make a drawing
showing:--

    (_a_) The muscles:

        (i) Two pectoralis muscles passing from the base of the arm, the
        one to the middle line of the body, and the other over the
        abdominal region to the posterior part of the body. Cut through
        this latter muscle at its anterior end, and fold it backwards to
        expose the underlying brachial vein.

        (ii) The rectus-abdominis muscles passing along the mid-ventral
        line of the body, with the white line (linea alba) in the
        centre.

        (iii) The deltoid muscle to the arm.

        (iv) The mylo-hyoid muscle, on the underside of the mouth region.

        (v) The obliquus-externus muscle, lying under the second
        pectoralis muscle noted above.

    (_b_) The cutaneous and the muscular veins, lying under the
    pectoralis muscle, and arising independently from the subclavian
    vein. They supply the skin and muscles in the region of the arm.
    There is a second cutaneous vein arising from the abdominal vein and
    extending on to the skin of the abdomen. Two other small cutaneous
    veins are seen on the skin of the leg, and another on the skin under
    the mouth.

    [_In the Frog a single musculo-cutaneous vein is seen lying under
    the pectoralis muscle and extends on to the skin._]

    (_c_) The abdominal vein, partly visible through the wall of the
    body, in the centre of the rectus-abdominis muscle.

    (_d_) The brachial vein going to the arm, near which may often be
    seen the white brachial nerve. Endeavour to trace the point of origin
    of the brachial, muscular, and cutaneous veins (_or the brachial and
    musculo-cutaneous veins in the Frog_) as shown in the figures.

[Illustration: FIG. 3.--Muscular System and Vascular System (part) of
Xenopus and Rana.]

2. Make an incision in the body wall, slightly to the right side of
the abdominal vein, which may now be seen clearly on the inner side
of the body wall. Carefully detach this vein from the abdominal
wall.


_Second Day._


=Skeletal System= (_Part_).

1. Remove the pectoralis muscles covering the body between the arms
to show the bones of the central part of the shoulder girdle and
sternum, viz.:--

    (_a_) The thick coracoids.

    (_b_) The more slender clavicles anterior to them, both extending
    from the base of the arm on each side towards the centre of the
    body.

    (_c_) The sternum, consisting of a median row of cartilages which
    are believed to be derived from the shoulder girdle. It does not,
    therefore, correspond to, or is not homologous with, the sternum of
    the Pigeon or the Rabbit, which is derived from the ribs. The
    sternum consists of the following parts:

        (i) The two epicoracoids consisting of cartilage.

        (ii) The metasternum which is broad and consists of cartilage
        only.

[Illustration: FIG. 4.--Pectoral Girdle of Xenopus and Rana.]

    [In the Frog the sternum is composed of the following parts:--
        (_i_) _The omosternum consisting of two parts, an anterior
        cartilaginous part (sometimes called the omosternum) and a
        posterior bony part (sometimes called the episternum)._

        (_ii_) _The two epicoracoids consisting of cartilage._

        (_iii_) _The metasternum consisting of two parts, an anterior
        bony part (sometimes called the sternum) and a posterior
        cartilaginous part (sometimes called the xiphisternum)._]

Make a sketch of the pectoral girdle. 2. Cut through the coracoids
and clavicles on each side, close to the base of the arm, and
carefully remove this part of the pectoral girdle and sternum so as
to show the heart lying underneath.

[Illustration: FIG. 5.--Dissection of Xenopus and Rana to show the
Alimentary System, etc.]


Alimentary System.

Make a sketch showing:--The heart, lungs, liver, gall bladder and
bile duct going to the duodenum, exposed parts of the stomach,
duodenum, small intestine (or ileum), large intestine, urinary
bladder, pancreas, and spleen.


Third Day.


=Vascular System= (_continued_).


A. Venous System.

1. Note:--

    (_a_) The heart lying in the thin pericardium, the two auricles, the
    single ventricle, the truncus arteriosus which divides into two
    branches.

    Remove the pericardium from the heart, and tilt the apex of the
    heart forwards, noting the sinus venosus into which the right and
    the left superior venæ cavæ (or precavals) and the single inferior
    vena cava (or postcaval) open.

    (_b_) The veins passing from each side of the sinus venosus. Each
    superior vena cava is joined by the following branches:--

        (i) The external jugular vein from the region of the tongue and
        the mandible.

        (ii) The internal jugular vein from the head and a thick
        subscapular vein from the shoulder. These veins join the
        superior vena cava independently of each other so that there is
        no innominate vein as in the Frog.

        (iii) The subclavian vein, made up of a brachial vein from the
        arm, a cutaneous vein from the skin, and a muscular vein from
        the abdominal muscles. The cutaneous and the muscular veins are
        separate veins so that there is no single musculo-cutaneous
        vein.

    [_In the Frog the superior vena cava is joined by the following
    branches_:--

        (_i_) _The external jugular vein, formed by the union of a
        lingual vein from the tongue and a mandibular vein from the
        margin of the lower jaw._

        (_ii_) _The internal jugular vein from the head and the
        subscapular vein from the back of the arm and shoulder, both
        joining the superior vena cava by a common vein, the innominate._

        (_iii_) _The subclavian vein, formed by the brachial vein from
        the fore-limb and a musculo-cutaneous vein from the muscles of
        the body and from the skin._]

    (_c_) The hypoglossal and the glossopharyngeal nerves, both running
    alongside of the lingual artery.

Make an enlarged drawing showing the above parts (_a_) and (_b_).

2. Note the inferior vena cava, receiving the renal veins from the
kidneys, the hepatic veins from the liver, and opening into the sinus
venosus.

3. Trace back the abdominal vein, and note that it is formed by the
union of the two pelvic veins, which pass along the bases of the
legs. The pelvic veins occur deep down in the body under the
posterior part of the pelvic girdle, which should be cut through in
order to see them. At the union of the pelvic veins to form the
abdominal vein a small rectal vein, passing along the ventral surface
of the rectum, joins the abdominal vein.

In order to make out the femoral and the sciatic veins, turn the
Platana over and remove the skin from the back of the leg. The
femoral vein may then be seen lying on the surface of the leg,
between two of the muscles. By carefully separating these muscles the
whole of the femoral vein may be exposed. Deeper down in the muscles
of the leg lies the sciatic vein, and, between the sciatic and the
femoral veins, the internal iliac artery may be seen running
alongside of the sciatic nerve.

Trace the femoral and the sciatic veins forward, and note that they
join together at the point where they meet the pelvic veins and then
run forward to the kidney as the renal portal vein on each side.

Turn the Platana over again on to its back and note the renal portal
vein from the ventral side. Joining the renal portal vein is a large
vein formed of two branches. The one branch arises in the abdominal
wall, while the other passes along the base of the leg and joins the
abdominal vein. On each side three dorso-lumbar veins join the renal
portal vein.

[Illustration: FIG. 6.--Venous System of Xenopus and Rana.]

[_In the Frog the abdominal vein is also formed by the union of the
two pelvic veins. The pelvic veins, however, lie more superficially
than in the Platana. The femoral vein which passes along the front of
the thigh divides into two branches, the one branch forming the
pelvic vein, the other, joined by the sciatic vein from the muscles
and skin of the back of the thigh, forming the renal portal vein
extending along the outer side of the kidney. The dorso-lumbar vein
from the dorsal wall of the body joins the renal portal vein._]

Make a drawing showing the above parts.


B. Arterial System.

Distend the œsophagus with a piece of cotton wool or paper to show
the blood vessels more clearly. Trace out the anterior part of the
arterial system, made up of two branches of the truncus arteriosus
each of which divides into:--

    (_a_) The carotid arch passing to the head region.

    (_b_) The systemic arch curving round the œsophagus.

    (_c_) The pulmo-cutaneous arch to the lungs and the skin.


_Fourth day._


=Arterial System= (_continued_).

1. Trace the carotid arch, which gives off:

    (_a_) Two lingual arteries having a common point of origin near the
    carotid gland.

    (_b_) A carotid artery passing to the head. At its origin it is
    slightly swollen into the carotid gland.

2. Trace the systemic arches which run backwards on each side of the
body, meeting posteriorly to form the dorsal aorta. Each systemic
arch gives rise to an artery which branches into:--

    (i) A scapular artery to the pectoral girdle.

    (ii) A cutaneous artery to the skin.

    (iii) A brachial artery to the arm.

Trace the dorsal aorta backwards, and note that it gives rise to the
cœliaco-mesenteric artery about midway between its origin and the
origin of the first pair of renal arteries. The dorsal aorta then
passes between the kidneys giving four renal arteries to each, and
proceeds backwards and divides into an internal iliac artery to each
leg. This artery lies between the femoral and the sciatic veins. From
each of these arteries a branch is given off which divides into:

    (i) An iliac artery to the muscles around the ilium.

    (ii) A cutaneous artery to the skin of the body, just anterior to
    the leg.

    (iii) A small artery to the anterior dorsal surface of the hind
    limb.

3. Trace the pulmo-cutaneous arch, which gives off:--

    (_a_) A pulmonary artery, running along the outer side of the whole
    length of the lung.

    (_b_) A cutaneous artery, which at first runs forwards to the base
    of the arm, and becomes spread out on the skin of the back of the
    head.

[_In the Frog_

    1. _The carotid arch gives off_:--

        (_a_) _A lingual artery supplying the tongue._

        (_b_) _A carotid artery, running round the side of the œsophagus,
        and supplying the head. At its origin it is slightly swollen into
        the carotid gland._

2. _The systemic arches run backwards on each side of the body, and
meet posteriorly to form the dorsal aorta. Each systemic arch gives
off_:--

    (_a_) _A subclavian artery to the shoulder and fore-limb._

    (_b_) _An occipito-vertebral artery to the head and the vertebral
    column._

    (_c_) _An œsophageal artery to the dorsal wall of the œsophagus._

    (_d_) _A laryngeal artery to the larynx._

    _At the point where the dorsal aorta is formed, note the origin of
    the cœliaco-mesenteric artery which sends branches to the stomach,
    liver, intestine, and spleen._

    _Trace the dorsal aorta backwards and note that it passes between
    the kidneys, to each of which it gives off four renal arteries and
    then it divides posteriorly into an iliac artery to each leg. The
    iliac artery is continued backwards as the femoral artery in the
    leg._

3. _The pulmo-cutaneous arch gives off_:--

    (_a_) _A pulmonary artery, running along the outer side of the whole
    length of the lung._

    (_b_) _A cutaneous artery, which at first runs forwards and upwards,
    and then turns backwards, supplying the skin of the back and the
    sides of the head._]

Make a complete sketch of the arterial system.

[Illustration: FIG. 7.--Arterial System of Xenopus and Rana.]

Cut out the heart, leaving attached to it as much as possible of the
large vessels.

Note the sinus venosus, truncus arteriosus, auricles, and ventricle.

Open the auricles and observe the division between them and the
openings of the veins passing into them.

Make a section through the auricles and ventricle noting the thick
muscular walls of the ventricle and the valves between it and the
auricles.


=Respiratory System and Alimentary System= (_continued_).

1. Remove the mylo-hyoid muscles to expose the hyoid apparatus. Pass
a seeker through the glottis to one of the lungs and slit it open
with a pair of scissors. Examine the structure of the lung.

2. Remove the whole of the alimentary canal with the lungs, liver,
pancreas, and spleen, by cutting through the œsophagus and the large
intestine before it joins the cloaca, and detaching it from the roof
of the body cavity without injuring the kidneys and the reproductive
organs.

Draw the complete alimentary canal.

[Illustration: FIG. 8.--Alimentary Canal of Xenopus and Rana.]


Urinogenital System.

Note the kidneys, two elongated, flattened, dark-red bodies lying one
on each side of the vertebral column towards the posterior end of the
abdominal cavity. From the outer edge of each kidney a ureter arises
and runs backward, opening into the cloaca on the dorsal side,
opposite the opening of the urinary bladder.

[_In the Frog the adrenal bodies may be seen on the ventral sides of
the kidneys as small yellow patches._]

In the male Platana note:--

    (_a_) The testes, a pair of ovoid pale-yellow bodies attached to the
    dorsal wall of the body cavity by a fold of the peritoneum. They lie
    on the ventral sides of the kidneys and are connected to them by
    efferent vessels which pass into the kidneys.

    (_b_) The corpora adiposa or fatty bodies lying in front of the
    kidneys.

    (_c_) The ureters which run along the outside of the kidneys and
    unite posteriorly to form a single duct opening into the cloaca.

[_In the Frog the ureters do not join together but open separately
into the cloaca. Each ureter has a slight swelling, the vesicula
seminalis, on its outer side._]

[Illustration: FIG. 9--Male Urinogenital System of Xenopus and
Rana.]

In the female Platana note:--

    (_a_) The ovaries, a pair of irregularly-lobed organs usually
    consisting of a mass of rounded black and white bodies, the ova. The
    ovaries are attached to the dorsal wall of the body cavity by a fold
    of the peritoneum.

    (_b_) The corpora adiposa lying in front of the kidneys.

    (_c_) The long convoluted oviducts opening into the body cavity in
    front and into the cloaca behind.

        The ureters open separately into the cloaca in the female.

Make a drawing of the urinogenital system.

[Illustration: FIG. 10.--Female Urinogenital System of Xenopus and
Rana.]


_Fifth Day._


=Skeletal System= (_continued_).

    A. The Pectoral and Pelvic Girdles and Limbs.

1. Detach a fore-limb with the remaining parts of the pectoral
girdle. Make a preparation showing these various parts:--

    Supra-scapula, scapula, remainders of the clavicle and the coracoid,
    humerus, radio-ulna, carpal bones, and digits. Draw these parts.

[Illustration: FIG. 11.--Pectoral Girdle of Xenopus and Rana.]

2. Detach the pelvic girdle from the transverse processes of the 9th
vertebra. Remove it with the hind-limbs from the body. Detach the
femur from one side of the pelvic girdle, and make a sketch showing
the following parts of the pelvic girdle:--Acetabulum, ilium,
ischium, and pubis.

[Illustration: FIG. 12.--Pelvic Girdle of Xenopus and Rana.]

Make a preparation and a sketch showing the femur, tibia-fibula,
astragalus, calcaneum and the other bones of the tarsal region, five
metatarsals, and digits. Number the digits from the inner side.

[Illustration: FIG. 13.--Hind and Fore Limbs of Xenopus.]


    B. The Vertebral Column.

1. Remove the skin and muscles from the dorsal surface of the back to
show the nine vertebræ and the urostyle. Make a drawing showing the
vertebræ with their transverse processes, and the urostyle attached
to the posterior end of the 9th vertebra.

[Illustration: FIG. 14.--Vertebral Column of Xenopus and Rana.]

The 2nd, 3rd and 4th vertebræ bear long transverse processes curved
backwards, those of the 3rd and 4th being tipped with small
cartilaginous ribs. The transverse processes of the 5th, 6th, 7th and
8th vertebræ are slender and project forwards and outwards. The 9th
vertebra has long flat wing-like processes attached to which are the
ilia of the pelvic girdle. The urostyle is fused to the 9th vertebra.

[_In the Frog the transverse processes are produced outwards and they
do not bear cartilaginous ribs. The urostyle is attached to the 9th
vertebra by two facets._]

Note the intervertebral foramina for the exit of the nerves from the
spinal cord, and the intervertebral discs.

2. Remove, examine, and draw the 1st or atlas vertebra, note its
ring-like form, absence of transverse processes, the concave facets
by which it articulates with the two occipital condyles of the skull.

3. Remove, examine, and draw the 3rd vertebra and note the
opisthocœlous centrum, neural arch and neural spine, anterior and
posterior zygapophyses, the transverse processes, and the
cartilaginous ribs.

[_The centrum is procœlous in the Frog and there are no ribs._]

4. Remove and examine the 8th vertebra and note that the centrum is
opisthocœlous. Make a drawing of a section showing the centrum.

[_In the Frog the centrum is amphicœlous._]

5. Remove and examine the 9th vertebra or sacrum, and note that the
centrum is slightly convex on the anterior surface. The urostyle is
fused to the posterior surface of the 9th vertebra. Note also the
long flat wing-like transverse processes.

[_In the Frog the centrum is also convex anteriorly and on the
posterior surface there are two convex facets for the articulation of
the urostyle._]


_Sixth Day._

=Skeletal System= (_continued_).

    C. The Skull.

1. Remove the skin and other tissues from the top of the skull and
observe the large fronto-parietals, nasals, supra-ethmoid,
premaxillæ, maxillæ, squamosals, exoccipitals, and pro-otics. The
quadrato-jugals are very inconspicuous. The extrastapedial plate of
cartilage is well marked and the bony columella may be seen lying
underneath it. The sides of the skull are ossified and may represent
the orbitosphenoids and alisphenoids of higher types. The
supra-occipital consists of a piece of cartilage.

[Illustration: FIG. 15.--Dorsal view of Skull of Xenopus and Rana.]

[_In the Frog there is no supra-ethmoid present, but a sphenethmoid
closes the front of the cranial cavity. The sides of the skull are
cartilaginous. A quadrato-jugal is well marked, but the quadrate
consists of a piece of cartilage at the posterior end of the
quadrato-jugal between the pterygoid and the squamosal. The lower jaw
articulates with it. The columella is usually not well shown._]

Make a drawing of the dorsal side of the skull.

2. Remove the skin from the roof of the mouth and note the
parasphenoid and single vomer, pterygoids, which are large and send
leaf-like processes towards the middle line covering the Eustachian
passages, squamosals, quadrates, basi-occipital, ex-occipitals,
premaxillæ, and maxillæ. The orbitosphenoid and alisphenoid are well
marked. There are no palatines present.

[Illustration: FIG. 16.--Ventral view of Skull of Xenopus and Rana.]

[_In the Frog there are a pair of vomers bearing the vomerine teeth,
palatines, and quadrato-jugals. The pro-otics, which are not seen in
a ventral view of the skull in the Platana, being covered over by the
pterygoids, are shown in the Frog._]

Make a drawing of the ventral side of the skull.

[Illustration: FIG. 17.--Side view of Skull of Xenopus and Rana.]

3. Make a drawing of the bones of the skull viewed from the posterior
end, showing the occipital condyle at each side of the foramen
magnum, ex-occipitals, basi-occipital, supra-occipital, pro-otics,
columella, and quadrate.

[_In the Frog the basi-occipital and supra-occipital are not
developed._]

4. Remove the skin from the lower jaw and make out the bones
surrounding the original Meckel's cartilage, viz., the mento-meckelian,
dentary, and articular.

[_In the Frog the lower jaw is composed of the mento-meckelian,
dentary and angulo-splenial._]

Make a drawing of the lower jaw.


Nervous System.

A. The Central Nervous System.

    The Brain.

Carefully remove the fronto-parietals so as to expose the brain, and
note the following parts:--

    (_a_) The olfactory lobes, forming the most anterior portion of the
    brain, united together in the middle line. Anteriorly the olfactory
    nerves are given off.

    (_b_) The cerebral hemispheres, a pair of smooth elongated bodies.

    (_c_) The thalamencephalon, lying immediately behind the cerebral
    hemispheres. It is covered by the choroid plexus, a thick vascular
    membrane, and gives rise dorsally to the stalk of the pineal body.

    (_d_) The optic lobes, a pair of oval bodies.

    (_e_) The cerebellum, a narrow transverse band immediately behind
    the optic lobes.

    (_f_) The medulla oblongata behind the cerebellum. It gradually
    tapers towards its posterior end, where it is continuous with the
    spinal cord. On its roof the pia mater forms a very vascular choroid
    plexus.

Make a sketch of the brain.


_Seventh Day._ (_Revision._)

With a second specimen revise the second, third, and fourth days'
work, paying particular attention to the vascular system.


_Eighth Day._


=Nervous System= (_continued_).

B. The Peripheral Nervous System.

    1. The Spinal Nerves.

Remove the abdominal viscera carefully so as to expose the spinal
nerves which are seen as white cords on each side of the vertebral
column as follows:--

    (_a_) The first or hypoglossal nerve arising between the 1st and 2nd
    vertebræ. It runs forwards on the under surface of the head beneath
    the mylo-hyoid muscle and supplies the tongue. It was noted in the
    third day's work.

    (_b_) The second and third nerves unite immediately after escaping
    from the spinal cord to form the brachial plexus, which supplies the
    muscles of the shoulder, and then extends along the arm.

    (_c_) The fourth, fifth, and sixth nerves are small and supply the
    muscles and the skin of the body wall.

    (_d_) The seventh, eighth, and ninth nerves run obliquely backwards,
    the eighth and ninth uniting to form the sciatic plexus, which gives
    rise to the large sciatic nerve which extends along the thigh, and
    down the leg. The sciatic plexus also receives a branch from the
    seventh nerve.

    (_e_) The tenth or coccygeal nerve, which escapes through a small
    aperture in the side of the urostyle, and is distributed to the
    walls of the cloaca and urinary bladder. It is connected by a branch
    with the sciatic nerve. This nerve is frequently small or absent in
    the Platana.

[_In the Frog, the seventh, eighth, and ninth nerves unite to form
the sciatic plexus which gives rise to the large sciatic nerve
extending along the thigh, and dividing above the knee into the
tibial and peroneal nerves to the leg and the foot. The tenth nerve
is present._]

    2. The Sympathetic Nervous System.

This system consists of a cord on each side closely following the
course of the systemic arches and the dorsal aorta. It has on it six
ganglionic swellings, and there are connections between the ganglia
and all the spinal nerves. Anteriorly each of the cords is joined to
the vagus nerve; it ends posteriorly as a broad swelling on the ninth
spinal nerve.

[_In the Frog the system consists of a chain of ganglia on each side
of the body closely following the course of the systemic arches and
the dorsal aorta. The cords have connections with the spinal nerves.
Anteriorly each cord ends in the Gasserian ganglion of the trigeminal
nerve and posteriorly it is united with the coccygeal nerve by a
varying number of branches._]

Make a complete sketch showing the spinal nerves and the sympathetic
nervous system.


    3. The Cranial Nerves.

Expose the brain and note the following cranial nerves:--

    (_a_) The olfactory nerve, arising from the anterior end of the
    olfactory lobes, supplying the olfactory capsule.

    (_b_) The optic nerve, arising from the ventral surface of the brain
    below the optic lobes, supplying the retina of the eye.

    (_c_) The trigeminal nerve, arising just behind the optic nerve,
    giving rise to the following branches:--

        (_i_) The ophthalmic nerve, which passes over the muscles of the
        eye and supplies the skin in the region of the olfactory
        capsule.

        (_ii_) The maxillary nerve, which runs forward under the
        eyeball, and supplies the skin covering the upper jaw.

        (_iii_) The mandibular nerve, which is at first continuous with
        the maxillary, and then bends round the inner surface of the
        upper jaw to extend along the outer surface of the mandible.

    (_d_) The glossopharyngeal and the vagus (or pneumogastric) nerves,
    leaving the skull together, immediately behind the auditory capsule.
    The glossopharyngeal passes along in front of the hypoglossal nerve
    and proceeds along the base of the tongue. The vagus, after giving
    off branches to the dorsal muscles, reaches the walls of the pharynx
    and run obliquely backwards to supply the pharynx, heart, lungs and
    stomach.

[Illustration: FIG. 18.--Brain. Cranial Nerves, Spinal Nerves, and
Sympathetic Nervous System of Xenopus and Rana.]

Make a complete sketch showing the brain and the above-mentioned
cranial nerves.


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Transcriber's Notes:

Every effort has been made to replicate this text as faithfully as
possible, including inconsistencies in formatting.

Italic text is denoted by _underscores_ and bold text by =equal
signs=.

Both "exoccipitals" and "ex-occipitals" occur in original text, left as
printed.





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