Text Book of Biology Part 16

There is no corpus callosum, nor is there a middle commissure to the third ventricle.

The cerebral hemispheres are not convoluted, and, looked at from the dorsal aspect, do not hide the thalamencephalon and mid-brain.

The pineal gland lies in the cranial wall and not deeply between the hemispheres, and its stalk is longer and tilts forward.

In the mid-brain--

The optic lobes are two, instead of being corpora quadrigemina, and hollow.

In the hind-brain--

The cerebellum is a very small transverse band, and has no lateral parts.

The medulla is relatively larger.

There are no spinal accessory nor hypoglossal nerves to the brain.

2. Compare the vertebrae of dog-fish, rabbit, and frog.

The centra of the dog-fish are -opistho- [amphi]-coelous (i.e., hollow at either end).

The centra of the rabbit are flat-faced.

The centra of the frog are procoelous (hollow in front).

The notochord persists between the centra in the dog-fish and rabbit, within the centra in frog.

The centra of the rabbit have epiphyses, absent in the dogfish and frog.

The transverse processes of the rabbit typically bear ribs.

Short ribs occur in the dog-fish, but their h.o.m.ology with those of the rabbit is doubtful. The frog has no ribs.

The interneural plates are peculiar to the dog-fish in this comparison.

3. Compare the skull of the dog with that of the frog.

The Brain Case--

Of the frog is a cylindrical box, from which the otic capsules project conspicuously on either side. It contains only two ossifications in its cartilaginous substance (the sphen-ethmoid and the ex-occipital), being protected by the membrane bones, the parieto-frontals above and the parasphenoid below.

In the mammal it is enormously inflated, and the otic capsules are imbedded in its wall. There are supra- and basi- as well as ex-occipital bones; the para-sphenoid is (? entirely) gone, and its place is taken by the basi- and pre-sphenoids, and the lateral walls contain fresh paired ossifications, the ali- and orbito-sphenoids-- all cartilage bones. The sphenethmoid is perhaps represented in part by the ethmoid.

As a result of the inflation of the brain-case, the squamosal, which slopes downward and outward in the frog, and overlies the cartilaginous suspensorium (quadrate cartilage), has become a const.i.tuent of the brain-case wall, and slopes downwardly and in.

Jaw Suspension--

The point of attachment of the jaw has shifted outward, and the original suspensorial cartilage (the quadrate) has taken on a new and minor function as the incus of the middle ear-- the squamosal superseding it as the suspensory part.

Lower Jaw--

Distinct bones in the frog; one ma.s.s in the dog.

Otic Capsule--

Position as specified. One centre of ossification in the frog forming pro-otic; several fuse together and form periotic of the dog.

There is no bulla and no external ear in the frog.

Palate--

In the frog the posterior nares open into the front of the mouth.

In the dog the maxillae and palatines send plates down and in (the palatine plates) to cut off a nasal pa.s.sage from the rest of the buccal chamber, and carry the posterior nares back to the pharynx, thus cutting the vomers off from the mouth roof.

The pterygoids in the dog are much reduced, and do not reach back to the suspensorium.

The frog has no lachrymal bone.

-Syllabus Of Practical Work_

We would impress upon the student at the outset the importance of some preliminary reading before dissection is undertaken. No one would dream of attempting to explore a deserted city without some previous study of maps and guide-books, but we find again and again students undertaking to explore the complicated anatomy of a vertebrated animal without the slightest, or only the slightest, preparatory reading. This is entirely a mistake. A student should be familiar with the nomenclature of the structures he contemplates examining, he should have some idea of their mutual relations and functions, or his attention will inevitably be diverted by the difficulty of new names and physiological questionings to the neglect of his dissection, and that careful observation of form and mutual position which is the essential object of dissection. On the other hand, it is equally necessary-- perhaps more so-- to warn students against the bookish fallacy, and to a.s.sure them of the absolute impossibility of realizing biological facts from reading alone. Practical work can alone confirm and complete the knowledge to which the text-book is the guide. In scientific teaching it may sometimes be convenient for the thought to precede the thing, but until the thing has been dealt with the knowledge gained is an unsatisfactory and unstable possession.

For such dissection as the subject-matter of this book requires, the following appliances will be needed:--

(a) Two or three scalpels of various sizes.

(b) Scissors, which must taper gradually, have straight blades, and be pointed at the ends, and which must bite right up to the tips (or they are useless). Two pairs, small and large, are advisable.

(c) Forceps, which must hold firmly, and meet truly at the points.

(d) Two needles set in wooden handles.

(e) An ordinary watchmaker's eye-gla.s.s is very helpful, but not indispensable.

(f) A dissecting dish-- an ordinary pie dish will do-- into which melted paraffin wax has been poured, to the depth of, say, three-quarters of an inch, and allowed to solidify. (This wax may be blackened by mixture with lampblack. If the wax floats up at any time, it can, of course, be remelted. Or it may be loaded with lead.)

(g) A rough table or board (for the rabbit and dog-fish).

(h) Blanket pins, and ordinary pins.

(i) A pickle or other wide-mouthed jar, and some common, methylated spirit.

(j) A microscope, with low power of 1 inch or 1/2 inch, and high power 1/6 inch or 1/4 inch. Gla.s.s slips and cover gla.s.ses, and a bottle of very weak (1 per cent.) solution of salt.

Animals for dissection may be obtained from the recognised dealers, who usually advertise in such scientific periodicals as Nature, Natural Science, and Knowledge. Sinel (naturalist, Jersey) is the most satisfactory dealer in dog-fish in our experience; Bolton (Malvern) will supply Amphioxus through the post; frogs and rabbits may be obtained anywhere. The tame variety of rabbit is quite satisfactory for the purpose of dissection.

The following notes may possibly be of some use to the student; they follow the lines of work arranged by the author for the evening cla.s.ses of the University Tutorial College, cla.s.ses considerably restricted as regards time, when compared with ordinary laboratory workers. Most of the sections below occupied about three hours, but for a student working alone they are more likely to take four or five, and even then it is not probable that they will be so satisfactory as if performed under skilled supervision. There are many points extremely difficult to convey verbally which are elucidated at once by actual demonstration upon a specimen. Each of these dissections should be repeated, and it is well if a different condition of the type is selected for the repet.i.tion-- an old one if the first specimen was immature, a female if the first was a male.