Injected and dissected so as to show its nervous, circulatory, and respiratory systems, together with some of its reproductive and digestive organs.
THE following external points may first be noted: the two slit-like external nostrils at the end of the snout, placed dorsally and widely separated: the large eyes partially covered by the thin moveable lower lid, the homologue of the third lid or nictitating membrane of higher Verte-brata, the upper lid being immoveable: the dark round smooth spot behind each eye where the outer skin adheres to the tympanic membrane: the pigmented skin with its innumerable minute vessels, injected in this specimen and correlated with the presence of the numerous glands characteristic of the soft moist transpirable skin of the Amphibia: the projections of the epidermis visible to the eye only when the specimen is held in certain positions, and best developed on the dorsum proper and the dorsal surface of the hind limbs: the cloacal aperture placed somewhat dorsally between the hind limbs: the short fore-limb: the base of the second finger (= index), dilated as it always is in the male at the breeding season: and the long hind limb with its webbed foot
The brain and abdominal viscera have been exposed by the removal of the skin, muscles, cranial roof, and left fore-limb. A space separates the skin from the underlying structures. It is divided into lymph sacs, fifteen in number, by vertical septa or by the union of the skin to the parts beneath. The two cerebral hemispheres of the brain are visible anteriorly; narrow in front where they pass into the olfactory lobes which are not exposed in this preparation, and broad behind where there is a diamond-shaped interval occupied in the centre by the base of the pineal gland and to either side by the optic thalami. Next come the large oval and obliquely placed optic lobes or corpora bigemina. A narrow transverse band behind the optic lobes represents the cerebellum. It leaves the large fourth ventricle or sinus rhomboidalis completely exposed.
The following structures in the body are visible from the ventral surface. Close to the angle of the lower jaw on the left side is an aperture, the passage to the croaking sac present only in the male. The heart lies medianly. It has been turned out of the pericardium, the thin membrane seen lying immediately below it. It consists of a conical yellowish opaque ventricle, separated by a well-marked auriculo-ventricular furrow from the thin transparent right and left auricle's. The conus arteriosus lies ven-trally between the auricles and is continued on into the truncus aortae which is extremely short in the Anura, the division of the Amphibia to which the Frog belongs. It appears to divide into a right and left half, each of which really consists of three vessels, carotid in front, aorta in the middle, pulmonary artery behind, none of them readily visible here. But the left vena cava superior may be seen between the root of the left lung and the left auricle where it dips down on its way to the sinus venosus. The liver with its right, central, and left lobes lies inferiorly to the heart. Some coils of the intestine are to be seen below the right lobe and leading downwards in the middle line from the median lobe is the epigastric vein.
At the lower end of this vein the left division of the bifid allantoid bladder projects sideways: the right division lies behind the vein with a small portion of the rectum visible just behind it. Turning the preparation so as to show the left side, the deeply injected left lung is observed to occupy the space between the left lobe of the liver and the cut edges of the dorsal skin and muscles. The oval yellow testis with black pigment speckled on its surface is placed dorsally, and posteriorly to the lung. At its lower end the vesicula seminalis projects touching the pylorus. The stomach and duodenum lie between the left and median lobes of the liver, and the lung and testis. The stomach tapers to the pylorus.
A remarkable hermaphrodite Nematode worm, Angiostomum nigrove-nosum ( - Ascaris or Leptodera nigrovenosa), is commonly found in the lungs. Its young pass through the intestine and become sexually mature in earth saturated with decaying animal matter. They are minute compared to their parent and their progeny wander back into the lung. A Trematode parasite, Polystomum integerrimum, with six posterior suckers and two large hooks, frequently occurs in the allantoid bladder: and the multi-nucleate Holotrichous Infusorian, Opalina Ranarum, inhabits the rectum.
The outer layers of the epidermis are shed periodically - a process which does not extend to the cornea of the eye. The skin-glands are of two kinds: (1) mucous glands with clear contents; (2) glands with granular contents, and probably poisonous like the similar glands in the Toad and Salamander. Chromato-blasts connected to nerves occur in the epidermis, but are most numerous in the outer layers of the corium. The concentration and diffusion of the black pigment within the cells depends on a reflex action of the nervous system set up through the eyes. In this manner the colour of the animal becomes adaptable more or less to its surroundings. The adult frog retains no trace of the organs of the lateral lines present in the tadpole. Besides free nerve-endings, the skin contains special 'touch spots' - flat cells lying near one another, each supplied by a nerve-filament.
The two olfactory lobes of the brain, generally so-called, are connected anteriorly by a transverse commissure: each lobe has a second root. They are solid, and it is probable that 'small papillae, situated at their base from which the olfactory nerves spring, and which contain a process of the lateral ventricle, should properly be regarded as the olfactory lobes. These papillae arise prior to the solid anterior prolongations of 'the hemispheres' (Balfour). The pineal gland is long and filamentous, pierces the cranial roof, and is attached to the skin at the level of a line drawn through the anterior angles of the two eyes. With the exception of the tips of the olfactory lobes, the several parts of the brain contain ventricles. The facial and auditory nerves have a common origin, and the ganglion of the former unites with the Gasserian ganglion. The ganglion of the glossopharyngeal is fused with that of the vagus, and the hypoglossus is the first spinal nerve. Of the latter there are ten in all. The anterior and posterior nerve-roots unite outside the neural canal. Round the ganglia, formed at the place of union, there lie sacs containing crystals of calcium carbonate - the so-called glands of Swammerdam. The Gasserian ganglion is similarly surrounded.