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 ; namely, two auricles and two 

 ventricles ; 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.). 



