The a-symmetry of the lungs is little more than a trifling difference of size between the two, dependent upon the encroachment of the heart by its displacement towards the left. And the a-symmetry of the great abdominal viscera enumerated above, and of their appen dages, depends entirely upon lateral displace ment and excesses of growth of one side over the other, having reference to conve nience of packing. This a-symmetry, greater in mammalia than in reptiles and fishes, on account of the presence of the diaphragm, which, so to speak, thrusts the abdominal viscera downwards, necessitating lateral dis placement, attains its acme in man, owing to the great lateral measurement compared with the antero-posterior distance which is so conspicuous in his figure when contrasted with that of other animals. The hepatic at tachments of the falciform ligament and gastro-splenic omen turn landmark the original median line of the liver ; and that larger part of it which is to the right of the ligament and behind the omentum, is the right lateral homologue of that lesser part which is to the left of the ligament and in front of the omen turn. The gall-bladder appears to be an un symmetrical organ, situated to the right of the median line. I am unable to state whether or not a left gall-bladder has once existed and been suppressed, or whether it is a diverticu lar production of the gall-duct evolved subse quently to the first sketching of the embryo ; or, lastly, whether it is originally median and subsequently displaced. The anterior wall of the stomach is the left lateral homologue of the posterior wall. The spleen is an ori ginally median organ, situated in the originally median meso-gastrium. The:great °mention is a pouching out of the meso-gastrium to wards the left, consequently its outer surface is the left lateral homologue of its inner sur face.* The pancreas is an originally median organ, one end of which has been displaced along with the pylorus towards the right, so that its anterior aspect is the left lateral ho mologue of the posterior. The intestinal canal can be witnessed in the embryo as a straight, uniform, mesial, and symmetrical tube, until its length having become greater than that of the region which it is destined to occupy, it is compelled to arrange itself in gyrations and loops. The posterior and an terior walls of the stomach were'originallv, as indicated above, its right and left halves; and as to the other parts of the alimentary tube, whatever difficulty there is in recog nising the manner of their displacement in the human subject, is at once dispelled by examining their condition in the lower animals. No difficulty whatever is encoun tered in respect of the small intestines, for their mesentery is attached nearly in the median line ; the bowels themselves, however, are continually varying their position, relative and positive, according to the manner of packing most convenient for their variable contents. Not so easy is it to understand the kind of displacement which has taken place in respect of the large intestines. The colon is curled back, and crosses over the small intes tine from right to left, forming a loop. In the
human subject, the true relation of these parts is further masked by the singular circum stance of this crossing over occurring just at the point where the meso-gastrium, after having descended as the great mental bag, is returning to the spine, and the colon, finding it there, so to speak, avails itself of it, and uses it as a mesentery ; anatomists have named this borrowed portion of the meso gastrium the transverse meso-colon. In Ru minants the colon, being exceedingly long, avails itself also of the mesentery of the small intestines, into which a loop of it is thrust further and further until it makes three turns ; so that in tracing the colon onwards with the finger, you make three spiral turns in the mesentery, and then double and return by three spiral turns placed between the former spirals.* On the other hand, in the Carnivora, where the colon is very short, it crosses over the lower end of the ileum so near to its termination that it is evident that the next degree of shortening must result in the continuation of the small and large intestines in a straight line. This actually takes place in the Reptilia t, and then there is no longer any difficulty in recognising the original mesial and symmetrical position of the intestinal tube and its appendages, so displaced in the human subject as to make this recognition so extremely difficult.
It is by no means uncommon to meet with instances where all the unsymmetri cally posited organs of the human body are placed completely vice versa to their usual situations; the heart pointing towards time right ; its pulmonic to the left of its systemic ventricle ; the vena cava to the left of the aorta ; the liver in the left, and the spleen in the right hypochondrium; and the csecum in the left iliac fossa. Almost every anatomical museum contains an instance of this kind ; and in all instances where the history of the case before death was known, the average health and ordinarily well-developed state of the subject attested the insignificance of this trans position in respect to the well-being of the individual. Such cases are most interesting from this consideration,--that all the unsym metrical organs are transposed. There is, I believe, no instance on record of one, or two, or less than the whole of the unsymmetrical organs occupying the side which is not their usual one. This prompts the belief that the side which these organs shall respectively oc cupy is determined by a single impulse first given to one of them.
Abnormal deviations from symmetry are of extremely frequent occurrence. The blood vessels of the body are very rarely perfectly symmetrical. In the adult subject the two sides of the body rarely match exactly in ex ternal form. The right hand is usually larger than the left. Accidental circumstances oc curring to an individual frequently disturb the symmetry, but it is by no means uncommon to meet with evidence of hereditary trans mission of aberrations from symmetry. Such monstrosities as supernumerary fingers and toes are sometimes symmetrical, but just as frequently, perhaps more frequently, the mon strosity exists on one side alone.