816 



SYMMETRY. 



ma}', however, be regarded as not sufficient 

 proof of this. The value to be given to such 

 facts depends, here as elsewhere, upon the 

 peculiar bias of different minds, and the asso- 

 ciations that pre-occupy them. It is not 

 necessary in the present state of our know- 

 ledge, nay, it is not expedient, to bind the 

 mind down to this or that view of the facts 

 that come before it. 



A remark, en passnn', is due to the considera- 

 tion that the corresponding organs and parts 

 of organs of the two sides are not only similar 

 in form and structure, but normally of exactly 

 the same age. The corresponding ossific points 

 of each side make their appearance at pre- 

 cisely the same time, and in all respects the 

 original development and subsequent evolu- 

 tion and maturation of the embryo proceed 

 exactly pari passu. The exceptions alone to 

 this rule of symmetry will chiefly occupy 

 our consideration. 



In Man, except perhaps in the very early 

 stages of his existence, exceptions are offered 

 by the heart, great blood-vessels, lymphatics 

 of the trunk, lungs, bowels, liver, spleen, and 

 pancreas, with their appendages. All these 

 parts were in all probability, in the very earliest 

 stages of the embryo, symmetrical ; most of 

 them have been proved by actual observa- 

 tion to have been so ; it is, however, difficult 

 to conceive this to have been the case with 

 the following other instances of departure 

 from symmetry. 



All decussations in the middle line are 

 a-symmetrical. Indeed, if a fibre crosses 

 the median line anyhow but at right angles, 

 there is a departure from symmetry. Con- 

 sequently, the decussation of tiie optic 

 nerves, the decussation of fibres in the me- 

 dulla oblongata, and the decussation of white 

 fibres at the linea alba, are instances of ex- 

 ceptions to symmetrical repetition. One is 

 tempted, in the last instance especially, to 

 believe that these decussations are not in the 

 original embryonic pattern or plan ; but that 

 they are developed subsequently, in obedience 

 to subsequent circumstances : the knowledge 

 of the power which circumstances, external 

 or internal, existing at any period of an 

 animal's life, have in modifying the directions 

 of fibres of areolar tissue, and of affecting it in 

 other ways, aids one in this belief. 



The most remarkable of the above instances, 

 and that in which the primordial symmetry is 

 most widely departed from, is the heart and 

 the great blood-vessels that are immediately 

 connected with it. The heart first makes its 

 appearance as a mass of cells posited in the 

 middle line, which soon becomes hollow and 

 divided into three compartments, the lower 

 one of which receives the embryonic veins, 

 and is therefore the auricle, whilst the upper 

 one is the commencement of the aorta, or 

 bulbus arteriosus. The middle cavity (the 

 ventricle) becomes bent into a horse-shoe 

 form, so as to bring the auricle and bulbus 

 into apposition. From the latter proceeds a 

 median artery, giving off six arches on each 

 side that surround the space occupied by 



the digestive canal and converge towards the 

 spine, where they are received by two sym- 

 metrical aortoe. The vein that enters the 

 auricle is a canal or sinus, that intercom- 

 municates between two venous trunks sym- 

 metrically placed on each side of the spine, 

 called ductus Cuvieri, the posterior continua- 

 tion of which is the cardinal (future azygos 

 and hemi-azygos) veins ; and the anterior, 

 the jugular. One of these ductus is permanent 

 as the superior cava. The omphalo-mesenteric 

 vein, which empties into the auricle, is at 

 this time situated in the middle line between 

 and in front of the cardinal veins, and into 

 it falls the single mesial vena cava clescen- 

 dens. The precise periods at which these 

 parts are first discernible, and those at which 

 the changes about to be described take place, 

 will be given in the article OVUM. It is 

 the manner of their metamorphosis alone 

 that bears upon this present inquiry. So far 

 all is symmetrical ; now commence those 

 changes whereby the adult impar arrange- 

 ment is eventually effected. The fourth and 

 fifth (from the heart) pairs of aortic arches 

 (the two anterior pairs), and the right one of 

 the second pair, soon disappear. The left one 

 of the second pair is permanent as the arch of 

 the aorta, and the third pair persist as the 

 subclavian arteries. The first pair give off 

 branches to the nascent lungs. Meantime, the 

 lower (abdominal) parts of the aortae have co- 

 alesced, and those parts of them which inter- 

 vene between the second arch on the left side 

 (the arch of the aorta) and the third (the left 

 subclavian), and between the first arch of the 

 right side (right pulmonary artery) and the third 

 (right subclavian), disappear. Now we have, 

 on the left side, the first two arches uniting 

 behind to form the left aorta. The first gives 

 off a branch to the lungs, and that portion of it 

 which is beyond this pulmonic branch remains, 

 as the ductus arteriosus, pervious till birth ; 

 whilst that portion which is nearest to the heart 

 remains, in connection with the pulmonic 

 branch, permanent for life as the left pulmonary 

 artery. On the right side the first arch alone, 

 the second having previously disappeared, forms 

 the right aorta, which soon joins its fellow 

 with which it coalesces below. The proxi- 

 mal part of this arch is permanent, and re- 

 mains in connection with its pulmonic branch 

 as the right pulmonary artery, whilst the part 

 of it beyond the pulmonic branch, together 

 with its continuation, the right aorta, down 

 to its point of junction with its fellow, soon 

 disappears. A septum, meanwhile, has been 

 developed in the ventricle of the heart, divid- 

 ing it into right and left ventricles, and an 

 imperfect one in the auricle. The bulbus 

 arteriosus also has been divided by a sep- 

 tum, in such a manner that the first pair of 

 arches remains in connection with one division 

 of it, which communicates with one of the 

 ventricles, whilst the permanent left arch of 

 the second pair communicates with the other 

 division, and through it with the other ven- 

 tricle. It is most probable that these septa, 

 when first developed, extend from side to 



