290 Proceedings of Boyal Society of Edinburgh. [april 2, 
actual development, and to deciplier slowly the laws she will not 
unveil to us at once. 
I fear I may weary you by these generalities, let me therefore 
show you by one particular example how human intelligence and 
nature may go in very different ways. 
The mammalian heart is described by anatomists as formed of two 
halves, a left and a right, each half consisting of an auricle and a 
ventricle. From the two ventricles arise the pulmonary artery 
and the aorta, two vessels which, in the embryo, and also in lower 
vertebrates, are united and form a contractile part of the heart. The 
aortic bulb Or arterial part of the ventricles and the auricular or 
venous part of the heart may be very early distinguished one from 
another, and the whole organ is at first a symmetrical tube, bifur- 
cated at each end, and consisting of a muscular and an endothelial 
wall. 
The heart-tube is formed by the union of a right and a left part. 
Notwithstanding this, it encloses at first a single cavity and the 
separation of two ventricles, two auricles and two great arteries, 
is of a later date. The separation begins with the ventricles ; the 
primary heart-tube is curved into a sling, the arterial and venous 
parts being fixed to the wall of the intestine, the ventricular part 
becoming free and twisted. By the twisting of the tube the arterial 
part comes to lie before the venous and to cross it, the ventricular 
part taking the form of a horse-shoe. The left part of the latter 
receives the auricular, the right one sends out the aortic blood. In 
the angle between the two branches the wall becomes folded, and 
forms an internal diaphragm. The separation by this first inter- 
ventricular wall is transverse to the axis of the tube, so that if it 
were to grow farther there would be a left ventricle without exit 
and a right one without entrance of blood, and circulation would be 
impossible. In reality, the opening between the two ventricles is 
only so far closed as to allow a free passage for the blood to and from 
both of them. It is effected by a series of complicated acts, and 
the whole septum cordis has no less than five origins, different not 
only in their primary position but also in the conditions of their 
formation. An account of the details of this development would be 
out of place here, as it would necessitate many drawings and techni- 
cal terms. Besides, the principle of the separation is easy to under- 
