VOL. LXXXV.] PHILOSOPHICAL TKANSACTIONS. 533 



that both sets make in opposite directions at the apex of the ventricle, fixes it and 

 prevents lateral motion. 



In the action of the ventricle, 2 different effects are produced ; the first brings 

 the apex nearer to the basis, by which means the vis inertiae of the blood is over- 

 come where the resistance is least, and a direction given to its motion in the 

 course of the aorta ; the 2d brings the sides nearer to each other, which ac- 

 celerates the motion of the blood already begun : and the spiral direction of the 

 fibres renders the power applied more uniform through the whole of that action, 

 than it could have been made by any other known form of muscle; the spiral 

 action also readily shuts the valvulee mitrales, while the apex is drawn up, which 

 could only be effected by this particular construction. By this beautiful me- 

 chanism the muscular fibres of the left ventricle of the heart perform their office 

 with a smaller quantity of contraction, compared to their length, though in 

 themselves proportionally longer, than those of any other muscle in the body, 

 and consequently produce a greater effect in a shorter time. 



The right ventricle is situated on the outside of the left, with which it is firmly 

 united ; it is not oviform in its shape, but triangular ; nor is it uniform in its 

 structure, being made up of 2 portions, whose fibres have a very different distri- 

 bution. The portion of this ventricle which makes a part of the septum of the 

 heart, consists of only one set of fibres, similar in their direction to those of the 

 stratum underneath, belonging to the left ventricle ; but from being considerably 

 shorter, they are more oblique than the spiral ; and at the edge of the cavity 

 they are blended with the fibres of the opposite portion. That portion which is 

 opposite to the septum is composed of 3 sets of fibres ; those of the external set 

 are nearly longitudinal ; the 2 others, which lie under it, decussate each other, 

 and are obliquely transverse in their direction, one passing a little upwards, the 

 other downwards ; and both terminate on the edge of the septum. 



In the structure of this muscle we find none of the mechanical advantages, so 

 obvious in the left ventricle; the want of these however is in some measure com- 

 pensated by its situation; for the blood contained in its cavity has the vis inertias 

 overcome, and a direction given to its course by the action of the apex of the 

 left ventricle : that motion only requiring to be continued, and accelerated, for 

 which purpose the structure of this muscle is very well adapted ; and in which it 

 is also assisted by the lateral swell of the septum into its cavity, in the contraction 

 of the left ventricle. 



In the course of this lecture, it has been my endeavour to show the most simple 

 structure that is capable of muscular action ; and to point out the advantages in- 

 tended to be produced by the different complications which occur in an animal 

 body. The view which I have taken of this subject gives us an idea of the ex- 

 tent to which muscular action is employed in different animals ; and leads to the 



