(a) the otic capsule becomes embedded in the brain-case wall;
(b) the palato-pterygoid rod lies completely underneath the brain-case instead of laterally to it;
(c) the squamosal tilts down and in, instead of down and out, and the lower jaw articulates with its outer surface instead of below its inner, and, moreover, with the enormous distention of the brain-case it comes about that the squamosal is incorporated with its wall.
(2) The maxilla anteriorly and the palatine posteriorly send down palatine plates that grow in to form the bony palate, cutting off a nasal pa.s.sage (n.p.) from the mouth cavity (m.p.), and carrying the posterior nares from the front part of the mouth, as they are in the frog, to the pharynx. Hence the vomers of the dog lie, not in the ceiling of the mouth, but in the floor of this nasal pa.s.sage.
Section 31. The quadrate cartilage of the frog is superseded by the squamosal as the suspensorium of the lower jaw. It is greatly reduced, therefore; but it is not entirely absent. In the young mammal, a quadrate cartilage can be traced, connected with the palato-pterygoid cartilage, and articulating with Meckel"s cartilage. Its position is, of course, beneath the squamosal, and just outside the otic capsule. As development proceeds, the increase in size of the quadrate, does not keep pace with that of the skull structures. It loses its connection with the palato-pterygoid, and apparently ossifies as a small ossicle-- the incus of the middle ear. A small nodule of cartilage, cut off from the proximal end of Meckel"s cartilage, becomes the malleus. The stapes would appear to be derived from the hyoid arch. Hence these small bones seem to be the relics of the discarded jaw suspensorium of the frog utilized in a new function. Considerable doubt, however, attaches to this interpretation-- doubt that, if anything, is gaining ground.
Section 32. The tympanic bulla of the dog is not indicated in Diagram 9, and it would appear to be a new structure (neomorph), not represented in the frog.
Section 33. Besides these great differences in form, there are important differences in the amount and distribution of centres of ossification of the skull of frog and mammal. There is no parasphenoid in the mammal*; and, instead, a complete series of ossifications, the median-, basi-, and pre-sphenoids, and the lateral ali- and orbito-sphenoids occur. The points can be rendered much more luminously in a diagram than in the text, and we would counsel the student to compare this very carefully with that of the Rabbit.
* Faint vestigeal indications occur in the developing skulls of some insectivora.
Section 34.
-Cranium_
-Nasal_ (paired), -Vomer_ (paired) -Fronto-Parietal_, Sphenethmoid Bone (median), Eye, Pro-otic Bone, Otic Cartilage, Ex-occipital (paired) -Para-sphenoid Bone_
-Upper Jaw_
-Pre-Maxilla_ (paired), -Palatine_ (paired), Pterygoid (paired), -Squamosal_, Quadrate Cartilage {To 1.} -Maxilla_ 1. Quadrato-Jugal
-Lower Jaw_
Mento-meckelian, -Dentary_, -Articulare- [-Angulo Splenial_]
Section 35. -Points especially- [Additional points] to be noticed are:
(1) The otic capsule (= periotic bone) of the dog ossifies from a number of centres, one of which is equivalent to the frog"s prootic.
(2) The several const.i.tuents of the lower jaw are not to be distinguished in the adult mammal.
(3) The frog has no lachrymal bone.
Section 36. We are now in a position to notice, without any danger of misconception, what is called the segmental theory of the skull. Older anatomists, working from adult structure only, conceived the idea that the brain-case of the mammal represented three inflated vertebrae.
The most anterior had the pre-sphenoid for its body, the orbito-sphenoids for its neural processes, and the arch was completed above by the frontals (frontal segment). Similarly, the basi-sphenoids, ali-sphenoids, and parietals formed a second arch (parietal segment), and the ex-, basi-, and supra-occipitals a third (occipital segment). If this were correct, in the frog, which is a more primitive rendering of the vertebrate plan, we should find the vertebral characters more distinct. But, as a matter of fact, as the student will perceive, frontal segment, parietal segment, and occipital segment, can no longer be traced; and the mode of origin from trabeculae and para-chordals show very clearly the falsity of this view. The vertebrate cranium is entirely different in nature from vertebrae. The origin of the parietals and frontals as paired bones in membrane reinforces this conclusion.
Section 37. But as certainly as we have no such metameric segmentation, as this older view implies, in the brain-case of the frog, so quite as certainly is metameric segmentation evident in its branchial arches. We have the four gill slits of the tadpole and their bars repeating one another; the hyoid bar in front of these is evidently of a similar nature; and that the ear drum is derived from an imperforate gill slit is enforced by the presence of an open slit (the spiracle) in the rays and dog-fish in an entirely equivalent position.
Does the mouth answer to a further pair of gill slits, and is the jaw arch (palato-pterygoid + Meckel"s cartilage) equivalent to the arches that come behind it? This question has been asked, and answered in the affirmative, by many morphologists, but not by any means by all.
The cranial nerves have a curious similarity of arrangement with regard to the gill slits and the mouth; the fifth nerve forks over the mouth, the seventh forks over the ear drum, the ninth, in the tadpole and fish, forks over the first branchial slit, and the tenth is, as it were, a leash of nerves, each forking over one of the remaining gill slits. But this matter will be more intelligible when the student has worked over a fish type, and need not detain us any further now.
Section 38. See also Section 13 again, in which is the suggestion that the occipital part of the skull is possibly a fusion of vertebrae, a new view with much in its favour, and obviously an entirely different one from the old "segmental" view of the entire skull, discussed in Section 36.
_Questions on the Frog_
[All these questions were actually set at London University Examinations.] {In Both Editions.}
1. Give an account, with ill.u.s.trative sketches, of the digestive organs of the common frog, specifying particularly the different forms of epithelium met with in the several regions thereof.
2. 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.
3. 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.
4. Draw diagrams, with the parts named, ill.u.s.trating 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?
5. 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-gla.s.s.
6. Describe the male and female reproductive organs of the common frog, and give some account of their development.
7. Describe, with figures, the bones of the limbs and limb-girdles of a frog.
8. Remove the brain from the frog provided, and place it in spirit. Make a lettered drawing of its ventral and dorsal surfaces.
9. 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.
10. (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.
11. 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?
12. 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?
13. Prepare the skull of the frog provided. Remove from it and place in glycerine on a gla.s.s 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.
14. 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.
15. 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.