Figure 1.

Phylogeny and scheme of the analysed structures of the avian hearts. (a) Phylogeny, on the basis of Jarvis et al. (2014), of the investigated orders. (b) In our investigations, we focused on the left and right atrium (LA and RA, respectively), the connecting veins, and the base of the left and right ventricle (LV and RV, respectively), and the arterial bases (see below for the number coding). In subsequent figure legends, we will refer to section numbers, where Section 1 represents the top horizontal line of the dashed box. (c) Schematic section in the transverse plane, which was the principle plane of sectioning, onto which we have collapsed all investigated structures: 1, myocardium in the sinus venosus; 2, the sinuatrial node; 3, sinuatrial valve and leaflets; 4, left sinus horn; 5, pectinate muscles of the atrial wall; 6, position of the right atrioventricular junction and muscularity of the valve; 7, position of the pulmonary artery and valve leaflets; 8, number of pulmonary veins; 9, the extent of pulmonary venous myocardium relative to the pericardium (dashed line); 10, dorsal ridge of the antechamber of the left atrium; 11, muscular shelf in the roof of the left atrium; 12, position of the left atrioventricular junction and muscularity of the valve; 13, position of the aorta and valve leaflets; 14, position of the orifices of the coronary arteries; 15, presence of ventral merger of the atrial walls