2r>2 DE. P. CHALMERS MITCHELL ON THE 



it is to l)e expected that the origin of that system from not more tlian three enterins: 

 nerve-hranches will he given an importance which as yet can only he suspected. The 

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

 only to a very limited numher of segments has an important hearing on medical work, 

 as it is heing found that there is an organic synipathy through the nervoiis system 

 hotween certain superficial areas of the skin and certain visceral organs or portions of th(^ 

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

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

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

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

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

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

 definite and equally metamerically 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 tiie other portions, and phylogenetically it is the oldest portion and corresponds to the 

 greatest numher of somites. It appears to he homologous throughout the vertebrate 

 series ; hut this homology depends on the identification of the paired caeca of Birds witli 

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

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

 bet\A een 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 rectiun ; 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 vUelli; but as it is obviously homologous with the caecum described 

 by Meckel as an abnormality in Man, and as in human anatomy it ])ears 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 22 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. Gladow (12), summing up his own very numerous 

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



