122 PROFESSOR OWEN ON THE ANATOMY OF 
the calibre of the intestinal canal gradually contracts to a circumference of one inch nine 
lines at the part which would be called jejunum in Man, and it recovers a circumference 
of three inches near the end of the ileum. The colon, within three inches of the ileum, 
has a circumference of nine and a half inches; and has decreased to a circumference of 
six inches, where it forms the rectum, about nine inches from the anus. 
The inner surface of the duodenum and jejunum is smooth, offering no villi to the 
naked eye. A few irregular very narrow longitudinal folds of the lining membrane, not 
parallel to, but following one another, begin to appear in the ileum: these are suc- 
ceeded by a longer longitudinal fold, or two, which are soon followed by one extending 
continuously through the ileum, along the side of the gut opposite the attachment of 
the mesentery: this fold is from two to three lines in breadth, is narrowest where the 
canal has been most distended, but is not obliterated by the utmost dilatation of the 
gut: it is a permanent single longitudinal production of the vascular lining membrane, 
and forms the chief characteristic of the lower half of the small intestines in the Myr- 
mecophaga jubata. In this part of the canal there are patches of glandule agminate 
from one to two inches long, and with intervals of about one foot. 
The transition of the ileum into the colon is effected by a rapid increase of diameter, 
viz. from one inch to two and a half inches; by a slight thickening of the muscular 
coat ; by the appearance of a few transverse ridges or very low folds of the mucous 
membrane at the beginning of the colon, and not extending round the circumference of 
the gut: but the boundary of the ileum is not defined by any ileo-colic valve nor by any 
appreciable alteration in the vascularity or other structure of the mucous membrane in 
the two divisions of the intestinal canal. 
The inner surface of the colon is smooth, finely reticulate, with a few very narrow 
transverse folds, from one inch to half an inch apart, subsiding for the most part before 
reaching the attached line of the gut ; these folds are not obliterated when the canal is 
fully distended ; they commence about eighteen inches from the ileum, gradually become 
shorter and narrower, and disappear about a foot from the rectum. 
The longitudinal folds of the rectum extend to the margin of the anus, where a little 
dark pigment is developed under the epithelium. The soft epithelial-covered integument 
extends from the fore part of the anus to the vulva, which is distant about half an inch. 
The longitudinal muscular fasciculi of the rectum and rectal end of the cloaca are 
strongly marked, and are from one line to one line and a half in breadth. 
In the thorax, a mediastinum, increasing in breadth from two to three inches as it 
passes backward from the aortic arch, completely divides that cavity into a right and 
left compartment ; the heart and pericardium projecting equally into both. A peculiar 
subcompartment of the right pleural cavity is formed by a duplicature of pleura 
extending from the right division of the mediastinum and from the lower part of the 
pericardium around the inferior cava, into which compartment the lobulus azygos from 
the right lung projects. 
