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fibres. In these diagrams, which I have taken from Dr. Sibson's ex- 

 cellent work on Medical Anatomy, a general view of the arrangement 

 of these fibres is shown. You see here represented what is called the 

 tendinous zone or tendinous ring. The fibres start from this tendinous 

 ring, wind round the heart in a spiral manner, and, having come to the 

 apex of the heart, enter it and run straight back again towards the 

 ring. In this model I endeavour to show you what I cannot show by 

 a diagram. Supposing this to represent the heart, here is the tendin- 

 ous ring which surrounds the great vessels that issue from the heart. 

 In order to avoid confusion, I have placed only four muscular fibres on 

 the model. Following the red one, you see it starts from this zone, 

 twines round the heart spirally, and, as you see in the diagram, makes 

 a complete revolution and comes back directly towards the spot from 

 which it started. It now leaves the outside surface of the heart and 

 enters the heart, and you may see it running almost in a straight line 

 up the inner side of the heart. The outer fibres, therefore, wind spirally 

 round the heart, enter it at the apex, and form, as they return to the 

 tendinous ring or zone from which they started, the lining of the internal 

 portion of the cavities of the heart. Imagine millions of such fibres 

 arranged in this spiral orbicular manner, as it was called by Borelli, 

 and you will have an idea of the complexity of the arrangements of the 

 heart. There is a necessary space left between these outer spiral fibres 

 and the returning fibres which you see running through the interior. 

 This space is filled in the heart with the proper fibres. The proper 

 fibres wind each in a spiral manner round their respective cavities and 

 go back again, so that we have three distinct groups of spiral fibres 

 arranged apparently in the most hopeless intricacy, but in reality accord- 

 ing to extremely simple and beautiful geometrical laws. The law which 

 regulates the arrangement of these fibres, so far as I have succeeded, in 

 discovering it, is this : the spiral fibre which goes round the entire of 

 the two cavities of the heart describes a complete circumference of 1 80° 

 before its return, whereas the spiral fibres that surround the right and 

 left ventricles of the heart respectively describe an entire circumference 

 and one- fifth over before they come back. This extra fifth of a turn 

 I believe is for the purpose of giving a twisting motion to the cavity, 

 just as you would wring a cloth, so that it should be completely 

 emptied at the close of the stroke and no blood left remaining in the 

 cavity, or even the least loss of force occasioned. 



I shall take this opportunity of publicly thanking Dr. Sibson for his 

 kindness in having placed at my disposal his unrivalled collection of 

 dissected hearts. He allowed me to take them with me to Dublin and 

 retain them for twelve months for study ; and he placed not only his 

 preparations but all his stores of knowledge at my disposal. As he is 

 not present, and therefore cannot blush at what I have to say, I will 



