

SYMMETRY 



851 



lorr 

 sim 



metry is definitely ascertained ; when it i.s de- 

 termined what is due to division, what to 

 radiation, or whether division and radiation, 

 one, or both, or neither, are concerned in their 

 causation. 



The skeleton of the Vclclla presents an in- 

 stance of departure'jfrom symmetry, mainly in 

 the oblique set of its vertical plate. The 

 common medusa has a circular outline, and 

 exhibits four quarters, which repeat one 

 another exactly, so that it seems to be marked 

 with a right-angled cross. The horizontal 

 plate of the skeleton of Velella is distorted to 

 a .sub-rhomboid form, and marked with two 

 diagonal seams that cross one another 

 obliquely. The longer one of these diagonals 

 is produced upwards so as to form the oblique 

 sail-supporting vertical plate, which also pre- 

 sents a seam continued up from the point of 

 crossing of the diagonals, indicating that it is 

 composed of two parts uniting at this central 

 point. This de facto unsymmetrical radiate 

 is therefore easily reduced in idea to its pri- 

 mordial symmetry. 



Infusoria. Out of the vast number of 

 various forms met with amongst these animal- 

 cules the greater number are symmetrical. 

 This symmetry is usually not bilateral, but that 

 of the star-fish. There are, however, some 

 forms which are quite irreconcilable with sym- 

 etry, which no line can divide into two 



ilar halves, which are one, and present no 

 repetitions of parts ; except, indeed, when 

 they are undergoing fissiparous generation. 



ANTERO-POSTERIOR SYMMETRY. By re- 

 ference to the diagram at page 824. Vol. III. 

 Fig. 433., representing the abstract notion 

 or type of a vertebral segment, it will be seen 

 that the upper or posterior half is a reverse 

 of the lower or anterior. Referring to what 

 really exists in nature, we find, in the caudal 

 vertebrae of fish, that their dorsal and ven- 

 tral halves are counterparts tolerably exact, 

 yet that exactness is not nearly what exists 

 between the lateral halves. If an antero- 

 posterior as well as a lateral symmetry be 

 admitted, then we have four repetitions ar- 

 ranged around a centre. At all events there 

 is here, in the Vertebrata, some amount of 

 evidence of radiation or divergence from a 

 central axis comparable in some degree to 

 what we see in the Radiata. The anterior 

 and posterior parts of vertebral segments, 

 as | found in nature, are usually extremely 

 dissimilar. The rays of the dorsal and ab- 

 dominal (anal) azygos fins of fishes are an- 

 tero-posterior repetitions, but the fan-like 

 tail fin, which in most fishes seems to be 

 remarkably symmetrical antero- posteriorly, 

 half of it apparently belonging to the dorsal 

 and the other half to the abdominal moiety 

 of the fish, usually belongs, as I have ob- 

 served in the typical fishes, cyprinoid, &c., 

 in reality, entirely to the abdominal or 

 under moiety. The embryo of these fishes 

 at first has a tail like an eel, into which 

 the spinal column is continued nearly to 

 its tip. A little way from the extremity of 

 this, on the abdominal aspect, a group of 



fin rays are soon observed to sprout out 

 meanwhile that the end of the vertebral co- 

 lumn shrinks and turns up. The group of 

 fin rays grows, and the vertebral column 

 shrinks, so that in time the former is brought 

 to form the fan-like extremity of the animal ; 

 but even then the atrophied end of the verte- 

 bral column may be detected occupying the 

 upper edge of the fan. Even in the adult 

 fish some trace of this original relation of 

 the tail fin can be detected. If the tail of 

 an adult hornocercal fish be macerated or 

 boiled, and all the pieces which are not an- 

 chylosed to it be removed, what remains will 

 not be symmetrical, but will terminate by 

 turning or cocking up. Even in the Pleuro- 

 ncctidee, whose tails seem to be remarkably 

 symmetrical, and where the spinal column 

 seems to terminate in a flat triangular piece, 

 it will be found that the lower half of this 

 piece can be easily removed, whilst the upper 

 half forms one piece with the body of the last 

 vertebra, with which, in fact, it forms a 

 coccyx composed of numerous degenerated 

 and consolidated vertebrae. 



SYMMETRY OF DISEASE. This subject has 

 been most ably treated by Dr. W. Budd *, 

 of Bristol. Those local diseases, the cause 

 of which is in the blood, usually affect the 

 solids of the body symmetrically. This can 

 be often well observed in lepra, psoriasis, se- 

 condary syphilis, gout, and rheumatism, and 

 in the eruption caused by taking the iodide 

 of potassium. It is due, no doubt, to the 

 symmetrical disposition of those tissues for 

 which the morbid poisons have a peculiar affi- 

 nity. It proves, moreover, that there is more 

 of peculiarity in certain parts of organs than 

 what meets the eye, which peculiarity is sym- 

 metrical. For, though the tissue of all parts 

 of a bone, for instance, shall be exactly the 

 same, it shall be diseased at a certain part 

 only, and the disease shall be repeated in ex- 

 actly the same part of the corresponding bone 

 of the opposite side. Connected with this is 

 the observation made above, that the corre- 

 sponding points of both sides of the body are 

 exactly of the same age have reached a cer- 

 tain stage of development at exactly the same 

 time. But the force of this observation in 

 explaining the symmetry of disease is consi- 

 derably weakened by another fact noticed by 

 Dr. W. Budd, namely, that the diseases above 

 named are apt also to localise themselves in 

 parts that are serially homologous, or corre- 

 sponding in the upper or lower limbs, as the 

 knee and elbow, wrist and ankle; for these 

 parts, though homologous, are not of the same 

 age. It is well known that the development 

 of the upper and lower limbs does not pro- 

 ceed pan passu. There is here, at least, some 

 determining agency more mysterious than 

 mere age. Still more curious and mysterious 

 is the relation of the eruption called shingles 

 to the bilateralism of the body. It often ex- 

 tends zone-like around one half, stopping 

 exactly at the median line before and behind, 



* Medico-Chirurgh-al Transactions for 1842. 

 3i 2 



