The left ventricle, ventriculus sinister, has the shape of a cone, the walls of which are 2–3 times as thick as the walls of the right ventricle (10–15 mm versus 5–8 mm). This difference is due to the muscular layer and is explained by the greater work done by the left ventricle (circulatory circle) compared to the right one (small circle). The thickness of the walls of the atria according to their function is even less significant (2 – 3 mm). The hole leading from the cavity of the left atrium to the left ventricle, octal ostium atrioventriculare sinistrum, is equipped with a left atrial-ventricular (mitral) valve, valva atrioventricularis sinistra (mitralis), of the two valves of which is smaller on the left and behind (cuspis posterior), – right and front (cuspis anterior). The free edges of the leaf are turned into the cavity of the ventricle, and chordae tendineae are attached to them. Musculi papillares are in the left ventricle, two in number – anterior and posterior; each papillary muscle gives the tendon threads of one or the other valvae mitralis valve. The aortic opening is called ostium aortae, and the nearest ventricular branch is conus arteriosus.
The aortic valve, valva aortae, has the same structure as the valve of the pulmonary trunk. One of the flaps, valvula semilunaris posterior, occupies the posterior third of the aortic circumference; the other two, valvulae semilunares dextra et sinistra, are the right and left sides of the hole. Nodules on their free edges, noduli valvularum semilunarium aortae, are more pronounced than on valves of the pulmonary trunk; lunulae valvularum semilunarium aortae are also available.
The septum between the ventricles, septum interventriculare, is represented mainly by muscle tissue, pars muscularis, except for the uppermost part, where there is only fibrous tissue covered on both sides with the endocardium, pars membranasea. Pars membranacea corresponds to the site of incomplete development of the interventricular septum of animals. There are often anomalies in the form of defects in the septum.