818 



SYMMETRY. 



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 unsymmetrieal 

 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. 



COMPARATIVE ANATOMY. I pass on now 

 hastily to notice such deviations from sym- 

 metry as are met with in the normal conditions 

 of the lower animals belonging to the ver- 

 tebrate sub-kingdom, and to examine the 

 question of symmetry in the other sub-king- 

 doms. 



In all Mammalia there is much the same 

 departure from symmetry in the viscera of the 

 chest and abdomen as is found in the human 

 subject ; but in no other mammal is the lateral 

 displacement so great as in man, for in all 

 others there is a greater proportionate antero- 

 posterior depth of chest and belly. The only 

 other notable instances of a-symmetry known 

 to exist in the mammalian class are the fol- 

 lowing: In the male narwhal or sea-unicorn 

 the left front incisor tooth attains the enormous 

 length of eight or ten feet, while the right one 

 is found as a rudiment that, never pierces the 

 gum. Spirals in the middle line are departures 

 from symmetry ; consequently the spiral penis 

 of the boar, &c. must be regarded as instances 

 of a-symmetry ; a slight excess in length of 

 one of the halves is sufficient to produce this 

 spiral form. The left nostril of most of the Ce- 

 tacea is constantly much larger than the right. 



In Birds there is much the same want of 

 symmetry in the viscera of the trunk as is met 

 with in mammals. The right one of the se- 

 cond pair of embryonic aortic arches however 

 is retained instead of the left, so that the adult 

 aorta arches over the right bronchus. But 

 notwithstanding this, the left ventricle is the 

 systemic one, and presents the same excess 

 over the right, in the thickness of its walls, as 

 in Mammalia. Both of the ductus Cuvieri are 

 retained and form two superior vence cavce. 

 The liver is situated mesially, that is to say, 

 its great fissure and falciform ligament are in 

 the middle line ; but its lift lobe is usually the 



larger, and in the common fowl presents a 

 fissure which is not repeated in the right. The 

 a'sophagus diverges slightly towards the right, 

 but the cardiac orifice of the stomach is to the 

 left of the pyloric. The long loop described 

 by the duodenum, and the pancreas which is 

 surrounded by it, are found extending diagon- 

 ally across the abdomen in front of the other 

 bowels, their extremity resting in the left iliac 

 fossa, but they are not fixed in this position. 

 The gall bladder is situated as in Mammalia. 

 There being no great omentum, the spleen 

 occupies its typical position behind the sto- 

 mach. There is usually a disparity in the 

 length of the pair of caeca met with in this 

 class. 



But the most remarkable exception to syni~ 

 metry in the class Aves is that which exists in 

 the female generative organs. The left ovary and 

 oviduct alone are functionally evolved, whilst 

 the right, becoming atrophied at an early 

 period, are barely traceable in the adult ani- 

 mal. A few instances are on record where 

 these right lateral homologues of the ovary 

 and oviduct have been found evolved in func- 

 tional size as a testicle and vas deferens, thus 

 forming lateral hermaphrodism. 



The male generative organs of birds are all 

 symmetrical except the penis, when it exists, 

 which is spiral. 



Another instance of want of symmetry is 

 presented by the beak of some birds, as the 

 cross-bill, &c. 



Reptiles. The heart of reptiles is situ- 

 ated in the middle line, but it is not symme- 

 trical in form, nor do the great blood-vessels 

 enter and quit it in a perfectly symmetrical 

 manner ; they however approach more nearly 

 to symmetry in this class than in any of those 

 which have been previously considered. In 

 all reptiles there are at first two aortse, sym- 

 metrically disposed, arching over the right 

 and left bronchi respectively, and uniting with 

 one another to form one trunk on the spine. 

 The vessels given off from these are, however, 

 most generally not symmetrical, the head and 

 front limbs being supplied frequently from the 

 right arch alone, and the chylopoietic viscera 

 from the left alone. The pulmonary arteries 

 arise behind the origin of the aorta?. The 

 lungs of reptiles are usually two symmetrical 

 organs, but in the Ophidk the left lung, 

 when it exists, is much shorter than the right, 

 and in some, as Coluber thiringicus, it is 

 wanting altogether, the only vestige of it 

 being a caecal depression on the left side of 

 the lower end of the trachea ; this absence of 

 the left lung entails, of course, the loss of the 

 left pulmonary vessels. 



The stomach and bowels of the Chelonia, 

 owing to the flattened form of the animals, 

 are nearly as much laterally displaced as in 

 the human subject. In the other reptiles 

 they are not much out of symmetry, yet in 

 none are they exactly symmetrical. The car- 

 diac end of the stomach tends, though often 

 but a little, towards the left ; the pyloric is 

 free, and can be brought without violence to 

 the middle line, but yet it is always found 



