TERATOLOGY. 



973 



example, the heart is often single, or even 

 shows signs of having been arrested in its 

 developement ; and, on the other hand, in the 

 more nearly single forms, the heart is usually 

 cither partially or completely double. The 

 histories of the cases of anterior and lateral 

 duplicity, which I have given in my mono- 

 graph, furnish abundant proofs of this. Nor 

 is there any closer relation between the con- 

 dition of any other internal organ and that of 

 the exterior, than there is between it and the 

 heart ; for in nearly complete external dupli- 

 city any of the internal organs may be single ; 

 when there are two trunks indeed, the urinary 

 and genital organs are commonly double ; but 

 as for the stomach, the liver, and the lungs, 

 the correspondence between them and exter- 

 nal duplicity follows no other rule than that 

 where there are two necks there are two 

 tracheae and two oesophagi, and as a con- 

 sequence the lungs and stomach are also dou- 

 bled. When in like manner the stomach is 

 double, each has its spleen and pancreas ; but 

 the state of the liver is very variable ; some- 

 times there are two, sometimes but one with 

 a single or double gall-bladder; and these 

 differences often occur in the same form of 

 external duplicity. 



Parts placed on the surface of the bod}' are 

 more liable to multiplication than the internal 

 organs, and duplicity of a single part is there- 

 fore much less rare than the formation of a 

 complete double body. The upper half of the 

 body is more frequently doubled than the 

 lower, probably in consequence of its earlier 

 developement and the admitted preponderance 

 of the upper parts of the body. The union 

 of the two bodies takes place only between 

 similar parts. The more each of the bodies 

 is developed, the less is the bond of union 

 between them, as the examples of the Siamese 

 twins and the Hungarian sisters sufficiently 

 prove. And with this law is connected an- 

 other, namely, that the probability of grow- 

 ing up is greater in the same proportion as 

 the bond of union is smaller, and the coin- 

 cident fusion of internal organs less, as these 

 two and other double monsters prove. So 

 also the further the several organs are 

 from the situation at which the bodies are 

 united, the more perfect they are, one 

 body is almost always less developed than 

 the other: in the heteradelphs this is al- 

 ways the case, and in others the difference 

 between the two bodies, though less evident, 

 is scarcely less constant than in them. There 

 are not commonly any signs of a double 

 monster having been at first two individuals. 

 For except in the cases of posterior and supe- 

 rior duplicity, and some singular examples of 

 attachment of the umbilical cord of one fetus 

 to the head or the body of the other, there is 

 never more than one placenta and one cord, 

 and the latter usually contains only a single 

 set of vessels, which divide when they reach 

 the abdomen. And even in the posterior and 

 superior varieties of duplicity it is not yet 

 certain that there are two placenta? ; in some 

 cases the placenta was positively single, and 



in the remainder it has very rarely been ex- 

 amined. A last general rule is that in double 

 monsters the twins are of the same sex. 

 There is no well proved exception to this 

 important rule. 



What explanation can be given, after all 

 these facts, of the origin of double monsters ? 

 On this subject, three hypotheses ought to be 

 mentioned : 1. The double monster has been 

 supposed to have proceeded from two distinct 

 embryos, which have become united in the 

 course of development ; 2. It has been held to 

 have originated in a single germ, which has 

 become double, or has been subdivided ; and 

 3. The germ has been regarded as abnormally 

 compound from the first, implying that the 

 organs and parts composing the double mon- 

 ster are at once produced from this germ, 

 without either separation or coalition of its 

 parts other than belong to the natural process 

 of developement. On the comparative merits 

 of the first and second hypotheses, as parts 

 of the general doctrine of monsters, one of 

 the most interesting physiological discussions 

 extant is recorded in the Memoires de FAca- 

 demie des Sciences de Paris, between 1724 and 

 1743. The chief disputants were Lemery 

 and Winslow ; the contest lasted nineteen 

 years ; it engaged the attention of all anato- 

 mists, and called forth writings by Haller and 

 a crowd of authors of less note, and was only 

 terminated by the death of Lemery. Every 

 argument that could be founded on the know- 

 ledge of those days was brought forward, and 

 the subject was, for the time, utterly exhausted ; 

 but the facts accumulated in later years have 

 furnished such volumes of additional evidence, 

 that the same question, between original and 

 acquired monstrosity, as far as it relates to 

 double monsters, may even now claim to be 

 discussed. 



It is certain that two ova may be formed 

 in one Graafian vesicle (Von Baer, Bischoff, 

 Bidder). The equally well-known fact, that 

 the common fowl sometimes produces double- 

 yolked eggs, naturally led at one time to the 

 opinion that the formation of double monsters 

 might be attributed to the developement and 

 subsequent union of the embryo in each yolk ; 

 an opinion which has been adopted by some 

 on very insufficient grounds, as it does not 

 appear to be warranted by any direct observ- 

 ations made upon the result of the incubation 

 of double-yolked eggs, and is at variance with 

 much of what is known of the structure and 

 mode of union of the two embryos composing 

 a double monster.* 



* I have here made use of the words of Professor 

 Allen Thompson, who, in a remarkable memoir upon 

 double monsters, published July, 1844, in the London 

 and Edinburgh Monthly Journal, fully entered into 

 the topic, and illustrated the genesis of double mon- 

 sters by some very interesting observations. I 

 lament that he was not acquainted with my mono- 

 graph upon the same subject, nor with the succinct 

 account which Mr. Paget has given of it in the 

 British and Foreign Medical Review for October, 

 1841. He would have found in it a great deal in 

 corroboration of his own opinions, in which I uni- 

 versally agree with him. 



