THE HUMAN HEART. 
359 
it, which is found in the perfect state of some of the lower in- 
vertebrata. It then attains the state of perfection in which it 
is found in fish, going no farther in that particular class ; but in 
Batrachians, after passing through the first two stages, develop- 
ment is not arrested as in fish, but goes on to a higher state ot 
perfection. In reptiles, the first three stages being passed 
through, advance is still made ; while in the heart of birds and 
mammalia, including man himself, the highest state of perfec- 
tion is at last reached only by passing through the others. 
Accordingly, the earliest form in which the heart presents 
itself is as a solid compact mass of embryonic cells, not differ- 
ing in themselves from the cells of which other organs of the 
body are constituted, since the cell is the primordial form in 
which essentially vitality resides, and of which all organized, 
bodies are entirely composed. At first there is no cavity in 
this heart, but shortly afterwards the cells in the centre seem 
to exert repulsive force on each other and become separated, 
thus forming a cavity which, however, is still closed ; a liquid 
next appears in the cavity, in which the central cells may be 
observed floating; but even before this, or before even the 
formation of a cavity, pulsation is observed to take place among 
the cells. To what such pulsation is owing is beyond our pre- 
sent, or perhaps our possible, knowledge ; the cells are similar 
to those in other parts of the body, and yet from their very 
earliest laying down in this position, and mutual relation, the 
function begins which the organ is to discharge during the 
whole period of existence. These pulsations are at first very 
slow, about fifteen to eighteen a minute, and they simply 
propel the contents of the cavity to and fro. So far, then, the 
heart is analogous to the first shadowing forth of a circulatory 
system which vve see in the lowest of the animal sub-kingdoms, 
the Protozoa, in whose transparent, gelatinous, celluliform 
bodies one or more clear pulsating spaces are observed in the 
interior of the cells, and which appear in some degree to effect 
a circulation in the soft substance of the body. 
The fluid within the cavity soon afterwards assumes the cha- 
racters of blood, having been at first a homogeneous fluid, like 
the circulating fluid in the class of insects. About the same 
time the cavity opens, forming communications with the great 
vessels in contact with it which have been developing them- 
selves pari passu, and subsequently the cells of which the walls 
of the heart are composed, are transformed into fibrous and 
muscular tissues, and into epithelium, which is a name applied 
to the cells which constitute lining membrane, whether exter- 
nally as the skin, or internally, as the mucous membrane. 
About the same time the heart, which was a straight cavity 
hitherto, becomes curved like a horse-shoe, and shortly after- 
