VASCULAR SYSTEM OF FROG 667 



the posterior region and from the gut passes through the 

 Uver on its way back to the heart, is as follows : — 



Hepatic portal 

 system. 



( 



Anterior abdominal vein, from the union of the 

 two pelvics, receiving tributaries from the 

 bladder, ventral body wall, and truncus 

 arteriosus. 



Hepatic portal vein, from the union of veins from 

 the stomach, intestine, and spleen. 



III. The pulmonary veins, which bring back purified 

 blood from the lungs, unite just before they enter the left 

 auricle. There are numerous valves in the veins of the 

 frog. 



Lymphatic system.— The lymph is a colourless fluid, hke blood 

 without red corpuscles. It is found in the spaces between the loose 

 skin and the subjacent muscles, in the pleuro-peritoneal cavity in which 

 heart, lungs, and other organs lie, in a sub-vertebral sinus extending 

 along the backbone, and in special lymphatic vessels which pass fatty 

 materials absorbed from the intestine into the venous system. There 

 are two pairs of contractile "lymph hearts" at two regions where the 

 lymphatic system communicates with the veins. A pair lie near the 

 posterior end of the urostyle ; the other two lie between the transverse 

 processes of the third and fourth vertebrae. Their pulsations can be 

 seen on the back of the living frog. 



Mechanism of the heart. — The right half of the ventricle, 

 being nearer the right auricle, contains more impure blo'od, and it is 

 from the right side of the ventricle that the conus arteriosiis arises. 



The middle of the ventricular cavity contains mixed blood. The 

 left corner contains pure blood received from the pulmonary veins. 

 The various valves and the conditions of pressure are such that the 

 venous blood passes by the pulmonary artery to the lungs, the next 

 quantum of blood enters the systemic arches, and the nearly pure 

 arterial blood from the left side of the ventricle passes into the carotids. 

 To understand the mechanism, it is necessary to consult some book 

 with a complete anatomical description, especially Gaupp's edition of 

 Ecker and Wiedersheim's Anatomie des Frosches (1899). 



Spleen, thyroid, and thymus. — The spleen is a small red 

 organ lying in the mesentery near the begimiing of the large intestine. 

 The thyroid is represented by two little bodies near the roots of the 

 aortic arches. The thymus, perhaps originally associated with the 

 gill-clefts, lies on each side just behind the angle of the lower jaw. 



Respiratory system. — The larval frog breathes at first 

 through its skin, then by gills. The adult frog breathes 

 chiefly by its lungs, but some cutaneous respiration is still 

 retained, for even without its lungs a frog may live for 

 some time, and it does not use them when hibernating. 



