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.