XI AMPHIBIA 411 



the form of adrenalin, an excess of which causes smooth muscle fibres, 

 such as those of the small arteries, to contract and in this way brings 

 about an increase in the pressure of the blood by increasing the peripheral 

 resistance to its flow. 



The heart of the Amphibians may be illustrated by that of the Frog 

 (Rana) which is commonly studied by the student of medicine both in 

 the course of Zoology and that of Physiology. 



The heart (Fig. 182) consists of the four usual segments : sinus 

 venosus — a thin-walled somewhat triangular chamber situated dorsally, 

 atrium — divided by a thin membranous partition into right and left 

 auricles (Fig. 182, A, r.a and l.a), ventricle (F) — undivided, and conus 

 arteriosus (C) — incompletely divided into systemic and pulmonary 

 portions by a longitudinal spirally twisted septum (s.c). 



The atrial portion of the heart has a thin wall strengthened by 

 bundles of muscle fibres. In the left auricle these tend to take on a 

 parallel arrangement, running longitudinally ; in the right they inter- 

 lace, running in all directions. More especially in the right auricle these 

 muscles tend to become free from the auricular wall except at their 

 ends, running straight across the cavity in the position of chords to the 

 curvature of the wall, instead of following the curvature throughout 

 their length. This modification in the position of the muscle bundles, 

 a common phenomenon in the evolution of the heart, clearly renders 

 them more effective. The interauricular septum (Fig. 182, A and C, s.a), 

 a thin, usually complete, membrane traversed by slender muscle bundles 

 and by two branches of the vagus nerve containing sympathetic fibres 

 (see p. 417), occupies an oblique position — the degree of obliquity differing 

 in different individuals — so that the right auricle extends ventrally well 

 over towards the left side of the heart. 



In a recess on the dorsal wall of the right auricle, a short distance 

 from the septum, is situated the opening from the sinus venosus in the 

 form of a transverse slit (s.o) guarded by the two sinu-auricular valves, 

 anterior and posterior. 



In the roof of the left auricle close to the attached edge of the septum 

 and slightly headward of the sinus opening is the rounded opening of 

 the pulmonary vein (j>.v). 



The cavities of both auricles are continued through the wide atrio- 

 ventricular opening into the undivided cavity of the ventricle. The 

 opening is guarded by four atrio-ventricular valves — two large (dorsal 

 and ventral — Fig. 182, C, a.v.D and a.v.V) and two small (right and left 

 — Fig. 182, C, a.v.R and a.v.L). Each valve is in the form of a thick 



