474 



DISSECTION OF THE THORAX. 



Sinuses of 

 Vaisaiva. 



Use. 



Structure of 

 the heart, 



Fibrous 



form rings 

 around auri- 



culo-ventn- 



cular 



and arterial 



Fibro-carti- 

 lage. 



The inter- 

 ventricular 



Special 



a hollow on the inner side, named sinus of Valsalva. The 

 cusps of the valve are an anterior and a right and left posterior in 

 relative position, and the right coronary artery arises in the anterior 

 sinus of Valsalva, and the left in the left posterior sinus. 



Like the valves in the pulmonary artery, these meet in the middle 

 to prevent the blood passing back into the ventricle, and combine 

 with them in causing the second sound of the heart. 



STRUCTURE. The heart is composed chiefly of muscular fibres, 

 together with certain fibrous rings and a fibro-cartilage. 



Dissection. The auricles should now be snipped round at their 

 junction with the ventricles, and the pulmonary artery and aorta 

 similarly cut round close to the attachments of the cusps of the 

 valves. The ventricular portion of the heart can then be cut 

 away and a view of the four valvular orifices obtained, and sections 

 should be made through them to demonstrate the fibrous rings 

 around the orifices. 



The fibrous structure forms rings around the auriculo-ventricular 

 and arterial orifices, and is prolonged into the valves connected 

 with these openings. 



The auriculo-ventricular rings give attachment to the framework 

 O f fibrous tissue in the tricuspid and mitral valves ; and the band 



* . . 



surrounding the lett auriculo-ventricular opening is blended in 

 front with the aortic ring. 



The arterial rings surround the aortic and pulmonary orifices ; 

 and the margin of each towards the artery forms three notches 

 with intervening projections. The notches are occupied by 

 thinner parts of the arterial wall bounding the sinuses of 

 Valsalva ; and to the concave edges the sigmoid flaps of the 

 valve are attached. 



Behind the aortic opening, between it and the auriculo- 

 ventricular apertures, is a piece of fibro-cartilage, with which the 

 fibrous rings are united. 



Dissection. The inter- ventricular septum should now be cut 

 through from below upwards. 



The interventricular septum appears as a stout pyramidal 

 muscular mass, between the two ventricles, but it will be seen that 

 the muscular tissue ceases close to the aortic orifice, and that, for a 

 short distance at the upper part, the ventricles are only separated by 

 a fibrous septum (pars membramacea septi). Sometimes a communi- 

 cation between the two ventricles occurs at this place, occasioning 

 one of the forms of congenital malformation of the heart. 



The STRUCTURE OF THE HEART beyond the stage already described 

 cann t be followed in the ordinary dissecting-room preparation, and 

 the further details can only be followed in a heart that has been 

 specially prepared. For this purpose a fresh heart is obtained 

 (commonly of a sheep or an ox), which, having been washed out, is 

 filled with a mixture of flour and water, and boiled for a quarter of 

 an hour, so as to destroy the connective tissue, and to allow the 

 stretched and hardened muscular fasciculi to be separated from 

 one another. 



