Text Book of Biology, Part 1: Vertebrata

by H. G. Wells

Section 1. We will now study the adult anatomy of the frog, and throughout we shall make constant comparisons with that of the rabbit. In the rabbit we have a distinctly land-loving, burrowing animal; it eats purely vegetable food, and drinks but little. In the frog we have a mainly insectivorous type, living much in the water. This involves the moister skin, the shorter alimentary canal, and the abbreviated neck (Rabbit, [Section 2]) of the frog; the tail is absent-- in a fish it would do the work the frog accomplishes with his hind legs-- and the apertures which are posterior in the rabbit, run together into one dorsal opening, the cloaca. There is, of course (Rabbit, [Section 4]), no hair the skin is smooth, and an external ear is also absent. The remarkable looseness of the frog's skin is due to great lymph spaces between it and the body wall.

Section 2. If we now compare the general anatomy of the frog (vide [Sheet 11]) with that of the rabbit, we notice that the diaphragm is absent (Rabbit, [Section 4]), and the body cavity, or coelom, is, with the exception of the small bag of the pericardium round the heart, one continuous space. The forked tongue is attached in front of the lower jaw, and can be flicked out and back with great rapidity in the capture of the small insects upon which the frog lives. The posterior nares open into the front of the mouth-- there is no long nasal chamber, and no palate, and there is no long trachea between the epiglottis and the lungs. The oesophagus is less distinct, and passes gradually, so far as external appearances go, into the bag-like stomach, which is much less inflated and transverse than that of the rabbit. The duodenum is not a U-shaped loop, but makes one together with the stomach; the pancreas lies between it and the stomach, and is more compact than the rabbit's. There is no separate pancreatic duct, but the bile duct runs through the pancreas, and receives a series of ducts from that gland as it does so. The ileum is shorter, there is no sacculus rotundus, and the large intestine has no caecum, none of the characteristic sacculations of the rabbit's colon, and does not loop back to the stomach before the rectum section commences. The anus opens not upon the exterior, but into a cloacal chamber. The urinary and genital ducts open separately into this cloaca, and dorsally and posteriorly to the anus. The so-called urinary bladder is ventral to the intestine, in a position answering to that of the rabbit, but it has no connection with the ureters, and it is two-horned.

Section 3. The spleen is a small, round body, not so intimately bound to the stomach as in the rabbit, but in essentially the same position.

Section 4. Much that we knew of the physiology of the frog is arrived at mainly by inferences from our mammalian knowledge. Its histology is essentially similar. Ciliated epithelium is commoner and occurs more abundantly than in the rabbit, in the roof of the mouth for instance, and its red blood corpuscles are much larger, oval, and nucleated.

Section 5. The lungs of the frog are bag-like; shelves and spongy partitions project into their cavities, but this structure is much simpler than that of the rabbit's lung, in which the branching bronchi, the imperfect cartilaginous rings supporting them, alveoli, arteries and veins, form together a quasi-solid mass.

Section 6. The mechanism of respiration is fundamentally different from that of the mammal. The method is as follows:-- The frog opens its anterior nares, and depresses the floor of the mouth, which therefore fills with air. The anterior nares are then closed, and the floor mouth rises and forces the air into the lungs-- the frog, therefore, swallows its air rather than inhales it. The respiratory instrument of the rabbit is a suction pump, while that of the frog is a "buccal force pump."

Section 7. The heart is not quadrilocular (i.e., of four chambers), but trilocular (of three), and two structures, not seen in Lepus, the truncus arteriosus and the sinus venosus, into the latter of which the venous blood runs before entering the right auricle, are to be noted. The single ventricle is blocked with bars of tissue that render its interior, not an open cavity, but a spongy mass. Figure 2, [Sheet 11], shows the heart opened; l.au. and r.au. are the left and right auricles respectively; the truncus arteriosus is seen to be imperfectly divided by a great longitudino-spiral valve (l.s.v.); p.c. is the pulmo-cutaneous artery -going to the lungs- [supplying skin and lungs]; d.ao., the dorsal aorta [furnishing the supply of the body and limbs]; and c.a. the carotid artery going to the head; all of which vessels (compare [Figure 1]) are paired.

Section 8. It might be inferred from this that pure and impure blood mix in the ventricle, and that a blood of uniform quality flows to lungs, head, and extremities; but this is not so. The spongy nature of the ventricle sufficiently retards this mixing. It will be noted that the opening of pulmonary arteries lies nearest to the heart, next come the aortic and carotid arches, which have a common opening at A. Furthermore, at c.g.l. [the carotid artery, repeatedly divides to form a close meshwork of arterioles, the carotid gland, forming a sponge-like plug in this vessel.] is a spongy mass of matter, the carotid gland inserted upon the carotid. Hence the pulmonary arteries yawn nearest for the blood, and, being short, wide vessels, present the least resistance to the first rush of blood-- mainly venous blood for the right auricle. As they fill up, the back resistance in them becomes equal, and then greater, than the resistance at A, and the rush of blood, now of a mixed quality passes through that aperture. It selects the dorsal aorta, because the carotid arch, plugged by the carotid gland, offers the greater resistance. Presently, however, the back resistance of the filled dorsal aorta rises above this, and the last flow of blood, from the ventricular systole-- almost purely oxygenated blood for the left auricle-- goes on towards the head.

Section 9. At the carotid gland the carotid artery splits into -an- [a] -external carotid- [lingual] (e.c.), and a deeper internal carotid. The dorsal aorta passes round on each side of the oesophagus, as indicated by the dotted lines in Figure 2, [Sheet 11], and meets its fellow dorsal to the liver. Each arch gives off subclavian arteries to the limbs, and the left, immediately before meeting the right, gives off the coeliaco-mesenteric artery [to the alimentary canal]. This origin of the coeliaco-mesenteric artery a little to the left, is the only asymmetry (want of balance) in the arterial system of the frog, as contrasted with the very extensive asymmetry of the great vessels near the heart of the rabbit. [Posteriorly the dorsal aorta forks into two common iliac arteries (right and left) supplying the hind limbs.]

Section 10. [Figure 3] gives a side view of the frog, to display the circulation.