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apex, which is its thinnest part. This condition may be explained by 

 a certain modification of the preceding description, by supposing, 

 namely (what is really the fact), that the outermost and innermost 

 layers extend further towards the apex and towards the base than 

 those which come next, and these again further than those which 

 succeed, and so on with the rest ; the central one being of least ex- 

 tent, and confined indeed to about the middle third of the ventricle. 

 In this way the ventricular wall is thickest towards its middle, where 

 it is composed of all the layers, but becomes thinner and thinner 

 towards the base and apex, where it consists of fewer and fewer 

 layers. 



Proceeding next to speak of the right ventricle, and especially of 

 its relation to the left, the Lecturer observed that the simplest way to 

 view that ventricle is to regard it as a segment of the left one ; and 

 this view he considers to be most in accordance with what we know 

 of its structure and mode of development. For a short time after 

 the heart appears in the embryo, its ventricular compartment is 

 simple ; but a septum soon begins to rise up within it, which pro- 

 ceeds from the right side of the apex and anterior wall of the cavity 

 in the direction of the base, and is completed about the eighth week 

 of intra-uterine life. For a time, moreover, the new-formed ven- 

 tricles have equally thick walls ; but as the full period is approached, 

 the left, which is destined after birth to perform a larger amount of 

 work, comes to predominate in thickness. Starting now from the 

 left or "typical" ventricle, constituted as above described, the 

 Lecturer showed that, by pushing in the anterior wall in imitation of 

 the constructive process in the embryo until it reaches the posterior 

 wall, two ventricles are produced, with a partition or septum between. 

 As, however, the septum in this case is double and unattached pos- 

 teriorly, he said it was necessary, in order to complete the structure, 

 to suppose the fibres forming the posterior border of the septal 

 duplicature as coalescing or anastomosing with corresponding fibres 

 of the posterior wall, whilst the fibres of the two halves of the dupli- 

 cature itself are blended with each other. In this way, as he ex- 

 plained, there results a single septum connected posteriorly, and 

 constituted in a manner which remarkably accords with the struc- 

 ture discovered by dissecting the adult heart. Thus, when both 

 ventricles are dissected at the same time, the fibres forming the 



