Picture - Cosmic Polymath

4o8 DESIGN IN NATURE
The circulation of mixed blood through the body of the fcetus resembles that of the reptile, where there are
two auricles and only one ventricle ; the right auricle containing venous blood and the left one arterial blood. As
the right and left auricles in the reptile communicate directly with the ventricle, the venous and arterial blood
are mixed in the ventricle and a mixed circulation produced. The foetal circulation resembles that of the fish to
the extent that the blood of the foetus is aerated not by coming in contact with air, but with the oxygen in solution
in the blood of the mother contained in the maternal portion of the placenta. The fish and foetus aerate their
blood by means of a fluid medium. In the fish the heart consists of a single auricle and ventricle, with two
accessory structures, the ductus venosus and the bulbus arteriosus, and the circulation is mixed because a perfect
circulation is impossible as apart from a heart with four separate compartments ; the two right compartments being
devoted to the venous circulation, and the two left compartments to the arterial circulation. All this does not make
the man successively a fish, a reptile, or a bird. It only shows that he is formed upon a common plan, and that
he forms one of several types. His distinguishing feature is his completeness.
The mixed circulation of the foetus is converted into a double or perfect circulation soon after birth ; the
Eustachian valve closing and the ductus arteriosus becoming imperforate. These structural changes are not due
to irritability, extraneous stimulation, or environment. The effect of them is to form a heart consisting of
four distinct and separate chambers ; a right and left auricle, and a right and left ventricle. The right auricle
and ventricle receive venous or impure blood from all parts of the system and force it into the lungs, and are, from
this circumstance, designated the pulmonic or right heart ; the left auricle and ventricle receive arterial or pure blood
from the lungs and force it through the entire system, and are termed the systemic or left heart. The left heart,
because of the excess of work to be performed by it after birth, becomes the more powerful and typical one.
In the foetus before birth the placenta equalises the amount of work performed by the right and left ventricles
respectively. The history of the formation of the heart affords a pregnant illustration of design.
At first it appears as a simple tube slightly expanded in the middle and with two vessels at either end. After
a time it throws itself into a spiral loop which overruns and is free at either end ; the free ends being continuous
with the blood-vessels which ultimately form the cavse and pulmonary artery and aorta. Subsequently the original,
looped, spiral tube is divided by duphcations or partitions into four compartments ;
ventricles ;
namely, two auricles and two
the auricles opening into each other by the Eustachian valve, and the pulmonary artery and aorta also
opening into each other by the ductus arteriosus (Plate xcv.. Pigs. 1, 3, 6, and 7, page 407). While these changes
are going on, a most intricate arrangement of the muscular fibres forming the auricles and ventricles, especially the
latter, is taking place. The arrangement of the muscular fibres in the left ventricle is exceedingly involved and
complex—so much so, that for over 200 years it formed the Gordian knot in anatomy. The muscular fibres are
arranged in four distinct spiral sets, namely, two external and two internal ; these being continuous with each other
and with other fibres at the apex and base of the ventricle. The two external sets wind spirally from the base to
the apex and form right-handed spirals. Conversely the two internal sets wind spirally from the apex to the base
and form left-handed spirals (Plate xvii.. Fig. 4, page 32). The two sets of spiral external fibres involute or turn
in at the apex, and the two sets of spiral internal fibres evolute or turn out at the base in such a way that the
external and internal fibres become continuous with each other both at the apex and the base of the ventricle.
Other deeper and more obhque spiral external and internal fibres, continuous with those already mentioned, occur.
It follows, that the several sets of external and internal spiral fibres cross each other at ever-increasing
angles until the centre of the muscular wall is reached. As a consequence of this arrangement, and as
I explained in 1858,i the spiral muscular fibres of the left ventricle may be separated into seven well-marked
layers ; the layers increasing in thickness from without inwards. Thus if the ventricle be dissected from the
outside, the two sets of spiral fibres forming the fijst layer are found to wind from left to right and from base to
apex of the ventricle in a nearly vertical direction ; the two sets of fibres forming the second layer winding in a
similar direction slightly more obliquely ; those of the third layer winding still more obliquely until the fourth or
central layer, where the fibres pursue a nearly transverse direction, is reached. When the fourth layer is exposed
everything is reversed ; the two sets of spiral fibres forming the fifth layer running from right to left and from apex
to base very obliquely, the two sets of spiral fibres forming the sixth layer pursuing a similar but less oblique direction ;
the spiral fibres forming the seventh or most internal layer also proceeding from right to left in a nearly vertical
direction. In virtue of this arrangement, the fibres of the first and seventh layers cross each other spirally at very
acute angles, those of the second and sixth layers crossing spirally at more obtuse angles, and those of the third
and fifth layers crossing spirally at still more obtuse angles. The fibres of the central layer, as explained, pursue
1 In this year my disseelious of the heart, over 100 in number, obtained the Senior Anatomy gold medal of the University of Edinburgh
awarded by tlie famous Professor John (ioodsir, i\i& facile 'priiuxps of anatomists. Tliey formed the subject of the Croonian Lecture of the Royal
Society of London m 1860 ; an abstract of the lecture being published in the Proceedings of the Society in the same year. Later, the lecture in
an extended form, and very fully illustrated, was assigned a place in the Philosophical Transactions (1864, Part iii.).