Dissection of the Platana and the Frog

First Day.

Cecil von Bonde

External Characters.

1. Make a drawing of the animal viewed from above, showing:—

(a) The nostrils.

(b) The eyes.

(d) The tympanic membrane, which is not shown in the Platana, but is well marked in the Frog.

(e) The opening of the cloaca.

(f) The limbs and their divisions.

(g) The skin with tube-like lines round the body. [In the Frog these lines are not present.]

Fig. 1.—Xenopus lÆvis and Rana fuscigula.

2. Make an enlarged drawing, about twice the natural size, showing:—

(a) The fore-limb with the digits.

(b) The hind-limb with the digits. The inner three digits have claws. [The claws are absent in the Frog.]

Name the parts of these limbs, and number the digits from within outwards, noting that the first is absent in the fore-limb.

Mouth Cavity.

Open the mouth cavity widely, and make a sketch showing:—

(a) The teeth on the upper jaw.

(b) There are no teeth on the vomer. [In the Frog each vomer is supplied with teeth.]

(d) The single opening of the Eustachian tubes. [In the Frog there is an opening on each side.]

(e) There is no tongue. [A tongue is present in the Frog.]

(f) The glottis.

(g) The gullet.

Owing to the complexity of the Eustachian tubes and the bony form of the ear cavity, it is not possible to pass a seeker from the Eustachian aperture through the tympanic membrane. [In the Frog the Eustachian tubes are short and straight and permit of this being done.]

Fig. 2.—Mouth cavity of Xenopus and Rana.

Muscular System and Vascular System (Part).

1. Pin the Platana on its back in water, and cut through the skin with scissors along the middle line, throughout the whole length of the body. Pin back the skin on each side, and make a drawing showing:—

(a) The muscles:

(i) Two pectoralis muscles passing from the base of the arm, the one to the middle line of the body, and the other over the abdominal region to the posterior part of the body. Cut through this latter muscle at its anterior end, and fold it backwards to expose the underlying brachial vein.

(ii) The rectus-abdominis muscles passing along the mid-ventral line of the body, with the white line (linea alba) in the centre.

(iii) The deltoid muscle to the arm.

(iv) The mylo-hyoid muscle, on the underside of the mouth region.

(v) The obliquus-externus muscle, lying under the second pectoralis muscle noted above.

(b) The cutaneous and the muscular veins, lying under the pectoralis muscle, and arising independently from the subclavian vein. They supply the skin and muscles in the region of the arm. There is a second cutaneous vein arising from the abdominal vein and extending on to the skin of the abdomen. Two other small cutaneous veins are seen on the skin of the leg, and another on the skin under the mouth.

[In the Frog a single musculo-cutaneous vein is seen lying under the pectoralis muscle and extends on to the skin.]

(d) The brachial vein going to the arm, near which may often be seen the white brachial nerve. Endeavour to trace the point of origin of the brachial, muscular, and cutaneous veins (or the brachial and musculo-cutaneous veins in the Frog) as shown in the figures.

Fig. 3.—Muscular System and Vascular System (part) of Xenopus and Rana.

2. Make an incision in the body wall, slightly to the right side of the abdominal vein, which may now be seen clearly on the inner side of the body wall. Carefully detach this vein from the abdominal wall.

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