As the demonstration has, in reference to the construction of the septum and of the right ventricle, been unavoidably disconnected, it is requisite to give a more systematic and com prehensive description of their particular for mation.
The septum is composed of three layers : a left, a middle, and a right layer. The two former properly belong to the left ventricle ; and the last or right layer exclusively pertains to the right ventricle. The two former are composed of the primitive mass of fibres de rived from the rope and the carnere columnw of the left ventricle; the left layer being formed of the expanded fibres, a, of the rope, a tt,f,g .280, in their first sweep round the cavity ; and the middle layer of the continued fibres of the rope in its second sweep, blended with the ex panded fibres of the two carnew columnw. These blended fibres form the extended layer its cut edge a applies itself to the cut edge b, evidently forming the middle layer of the septum. The last or right layer of the septum has not the same origins as the two former have. Its fibres arise from the root and lower margin of the valve of that section of the aorta which pertains to the right ven tricle, from that part of the root of the pulmo nary artery contiguous to the aorta, and from the carnew eolumnte of the right surface of the septum. The fibres attached to the aorta and pulmonary artery may be seen infig.278, lettered A and a respectively, and in fig. 280 the fibres from the aorta blended with those of the car new columnw are exhibited marked Ace, forming the right layer of the septum.
The right ventricle.—Althougli the right layer of the septum belongs anatomically to the right ventricle, yet when functionally considered it pertains, as well as the other layers, entirely to the left. For the concavity of this layer is, like that of the other layers of the septum, to wards the cavity of the left ventricle, and therefore during the systole approaches the axis of this cavity, while it recedes from that of the right ventricle ; thereby assisting in the propul sion of the blood from the former, and to a limited extent counteracting the propulsive effort of the latter ventricle.
The right ventricle has, therefore, but one proper wall, which is connected to the left ventricle in a manner to be described hereafter. The right chamber should be divided into three channels : the auricular, the pulmonary or ventricular, and the apicial. The auricular is
that which receives the blood directly from the right auricle ; the pulmonary is that formed by the fibres which arise from the root of the pulmonary artery at its entire circumference : in fig. 278, the pulmonary artery, PP, and the fibres, e, are seen turned a little upon their axis, by which means the fibres are rendered oblique, and the channel the more complete; and the apicial channel is that which forms the channel of communication between the other two, and which extends to the apex. The pro per wall is considered as having three layers, the superficial, middle, and internal, although they cannot always be detached from each other. The superficial is composed of the mere superficial fibres of this wall, having the same origins and terminations as have its sub jacent fibres; it forms the left wing Citce of fig. 279, and may be seen in fig. 281, raised from the right ventricle and reflected over the base marked CACC. The middle layer is composed of two bands, the apicial and the basial. The apicial is formed of the first semi circular portion of the shorter band of the heart, and passes over the lower half or apicial channel of this chamber ; it lies separated and extended over the apex of fig. 281, marked CACC. The basial band of this layer is formed of the first semicircular portion of the longer band as it makes its second circle round the heart. It is bisected and separated as seen at CPCAA, of fig. 281 ; in its natural situation it passes over the pulmonary and auricular channels of this ventricle, and is closely connected to the base. The internal 2 r 2 layer arises chiefly from the pulmonary artery, PP ; it first forms the pulmonary channel, p, and then expands into a layer which crosses obliquely over the apicial channel, associated with fibres derived from one of the carnein coluninie. The basial portion of this layer which crosses over the auricular channel, can not often be separated from the fibres of its superjacent band, the fibres of the musculi pectinati being intricately interwoven with them. When this layer is replaced, its lower loose edge applies itself to the anterior boun dary, a b, of this cavity, and is lined with its internal proper membrane. Of the three layers composing the proper wall of this ventricle, two, the middle and inner layers; are confined at the edge of the septum, forming thereby the lateral boundary of this cavity.