SYMMETRY. 



847 



side, consequently that the systemic ventricle 

 anil aorta are at first in front of the pulmo- 

 nary ventricle anil artery, and that the heart 

 subsequently undergoes a twist towards the 

 right, carrying the systemic ventricle round 

 behind to its permanent position on the left. 

 In reptiles, in which a great many other of the 

 mammalian foetal characters are permanent, the 

 root of the systemic is in front of that of the 

 pnlmonic artery, and there is not that twining 

 round one another of the great arterial trunks 

 which is met with in man. Thus, then, these, 

 the most unsymmetrical parts of the whole 

 body, can be proved by actual observation to 

 have been originally perfectly symmetrical, 

 and the manner of their attainment to their 

 ultimate unsymmetrical form can be accu- 

 rately traced. 



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. 

 Ami 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 omentum 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 anil 

 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 omentum 

 is a pouching out of the meso-gastrium to- 

 wards the left, consequently its outer surface 

 is the left lateral homologne 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 embrvo as 

 a straight, uniform, mesial, and symmetrical 



* See PERITONEUM. 



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 originally, 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 nearlv 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 omental 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 hop 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 -\-, 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- 

 cal ly posited organs of the human body are 

 placed completely vice versa to their usual 

 situations; the heart pointing towards the 

 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 caecum 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 



f 



RUMINASTIA. 



-lyi. Yul. III. PKKITONEL'.M. 



