202 DE. P. CHALMERS MITCHELL ON THE 



it is to be expected that the origin of that system from not more than three entering 

 nerve-branches will be given an importance which as yet can only be suspected. The 

 nature of Meckel's tract as a recent outgrowth of a portion of the gut corresponding 

 only to a very limited number of segments has an important bearing on medical work, 

 as it is being found that there is an organic sympathy through the nervous system 

 between certain superficial areas of the skin and certain visceral organs or portions of the 

 organs ; the sympathy depending on a common relation to the primitive segmentation of 

 the body, and being of practical value in the diagnosis of affections of internal organs. 

 In those creatures in which there is a yolk-sac or umbilical cord, the outgrowth of 

 Meckel's tract from the primitive straight gut is opposite the vessels running to these 

 structures ; and it seems tempting to regard the origination of Meckel's tract as being 

 dependent on this mode of embryonic nutrition. However, as Meckel's tract is equallv 

 definite and equally rnetamerically limited in the Frog, we cannot lay much stress on 

 this possible mode of origin. 



The large intestine, from the caeca to the cloaca or anus, is the third distinct portion of 

 the intestinal tract. It always lies in the primitive position of the straight gut, dorsad of 

 all the other portions, and phylogenetically it is the oldest portion and corresponds to the 

 greatest number of somites. It appears to be homologous throughout the vertebrate 

 series ; but this homology depends on the identification of the paired cadca of Birds with 

 the unpaired cascum of Mammals and Lizards a homology not apparent when a bird 

 with a short gut is taken, but much more convincing when the comparison is made 

 between a form such as Palamedea (fig. 1) or a Struthious bird and a Mammalian or 

 Lacertilian intestinal tract. In the more primitive types the large intestine is very long, 

 and may show traces of division into colon and rectum ; in the higher forms, partly in 

 correlation with the greater development of the thoracic portion of the viscera, the 

 duodenum and whole intestinal tract has shifted far distally, with the result that the 

 large intestine is extremely small, and is here referred to simply as rectum (see footnotes, 

 pp. 176 and 271). 



MECKEL'S DIVERTICULUM. 



Examination of the embryo of any bird makes it plain that this structure is the 

 vestige of the yolk-sac, and its retention in adult birds has been described by a number 

 of anatomists. It has been termed by most writers who understood its nature the 

 Diverticulum ccecum vifelli; but as it is obviously homologous with the caBcuin described 

 by Meek el as an abnormality in Man, and as in human anatomy it bears the well-known 

 name of Meckel's diverticulum, I have preferred to give it that name in birds. I have 

 not found it present in any adult reptile, and I can find no record of its occurrence 

 there. Nor have I found it in any mammal other than Man. In human anatomy its 

 frequency of occurrence has received recent attention, and Birmingham (4) states that it 

 is present in about 2'2 per cent., varying from half an inch to five inches in length. In 

 birds it is present much more frequently ; indeed, its presence throughout life is a 

 character of very many groups. Gado\v (12), summing up his own very numerous 

 observations and those of earlier writers, states that it is retained throughout life by 



