2. _Questions on the Frog_
[All these questions were actually set at London University Examinations.] {In Both Editions.}
- Give an account, with illustrative sketches, of the digestive organs
of the common frog, specifying particularly the different forms of
epithelium met with in the several regions thereof.
- Describe the heart of a frog, and compare it with that of a fish and
of a mammal, mentioning in each case the great vessels which open
into each cavity.
- Compare with one another the breathing organs and the
mechanism of respiration in a frog and in a rabbit. Give figures
showing the condition of the heart and great arteries in these animals,
and indicate in each case the nature of the blood in the several
cavities of the heart.
- Draw diagrams, with the parts named, illustrating the arrangement
of the chief arteries of (a) the frog, (b) the rabbit. (c) Compare briefly
the arrangements thus described. (d) In what important respects does
the vascular mechanism of the frog differ from that of the fish, in
correlation with the presence of lungs?
- In the frog provided, free the heart, both aortic arches, dorsal aorta
as far as its terminal bifurcation, and both chains of sympathetic
ganglia from surrounding structures; and remove them, in their
natural connection, from the animal into a watch-glass.
- Describe the male and female reproductive organs of the common
frog, and give some account of their development.
- Describe, with figures, the bones of the limbs and limb-girdles of a
frog.
- Remove the brain from the frog provided, and place it in spirit. Make
a lettered drawing of its ventral and dorsal surfaces.
- Point out the corresponding regions in the brain of a frog and a
mammal, and state what are the relations of the three primary
brain-vesicles to these regions.
- (a) Give an account, with diagrams, of the brain of the frog; (b)
point out the most important differences between it and the brain of
the rabbit. (c) Describe the superficial origin and the distribution of the
third, (d) of the fifth, (e) of the seventh., (f) of the ninth, and
(g) of the tenth cranial nerves of the frog.
- Describe, with figures, the brain of a frog, and compare it with that
of a rabbit. What do you know concerning the functions of the several
parts of the brain in the frog?
- Describe briefly the fundamental properties of the spinal cord in
the frog. By what means would you determine whether a given nerve
is motor or sensory?
- Prepare the skull of the frog provided. Remove from it and place in
glycerine on a glass slip the fronto-parietal and parasphenoid bones.
Label them. Mark on the skull with long needles and flag-labels the
sphenethmoid and the pro-otic bones.
- Compare the skull of the rabbit and the frog; especially in regard
to the attachment of the jaw apparatus to the cranium, and other
points which distinctly characterize the higher as contrasted with the
lower vertebrata.
- Describe the skeleton of the upper and lower jaw (a) in the frog, (b) in the rabbit. Point out exactly what parts correspond with one another in the two animals compared. (c) What bone in the rabbit is generally regarded as corresponding to the quadrate cartilage of the frog?
-The Dog-Fish._
1. _General Anatomy._
Section 1. In the dog-fish we have a far more antique type of structure than in any of the forms we have hitherto considered. Forms closely related to it occur among the earliest remains of vertebrata that are to be found in the geological record. Since the immeasurably remote Silurian period, sharks and dog-fish have probably remained without any essential changes of condition, and consequently without any essential changes of structure, down to the present day. Then, as now, they dominated the seas. They probably branched off from the other vertebrata before bone had become abundant in the inner skeleton, which is consequently in their case cartilaginous, with occasional "calcification" and no distinct bones at all. Unlike the majority of fish, they possess no swimming bladder-- the precursor of the lungs; but in many other respects, notably in the uro-genital organs, they have, in common with the higher vertebrata, preserved features which may have been disguised or lost in the perfecting of such modern and specialized fish as, for instance, the cod, salmon, or herring.
Section 2. Comparing the general build, of a dog-fish with that of a rabbit, we notice the absence of a distinct neck, and the general conical form; the presence of a large tail, as considerable, at first, in diameter as the hind portion of the body, and of the first importance in progression, in which function the four paddle-shaped limbs, the lateral fins, simply co-operate with the median fin along the back for the purpose of steering; and, as a consequence of the size of the tail, we note also the ventral position of the apertures of the body. The anus, and urinary and genital ducts unite in one common chamber, the cloaca. Behind the head, and in front of the fore fin (pectoral fin), are five gill slits (g.s.) leading from the pharynx to the exterior. Just behind the eye is a smaller and more dorsal opening of the same kind, the spiracle (sp.). On the under side of the head, in front of the mouth, is the nasal aperture (olf.), the opening of the nasal sac, which, unlike the corresponding organ of the air-frequenting vertebrata, has no internal narial opening. There is, however, a groove running from olf. to the corner of the mouth, and this, closing, in the vertebrate types that live in air and are exposed to incessant evaporation of their lubricating secretions, constitutes the primitive nasal passage. The limbs are undifferentiated into upper, lower, and digital portions, and are simply jointed, flattened expansions.