74 DESCRIPTIONS OF PREPARATIONS. 



14. COMMON FROG (Rana temporaria), 



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 charac- 

 teristic 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 trans- 

 verse 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 auricles. 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 



