  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. [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. [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. 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. |
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