DIGESTIVE SVSTEM 139 
as well. The structure of these small but important organs will be 
best understood by reference to the accompanying figure. Each 
villus consists of a finger-shaped prolongation of the tissue of the 
submucosa, which contains a ramified central canal conveying the 
collected chyle into the lymphatic vessels, which are frequently 
connected with a lymphatic follicle for the production of white 
BLOOD-corpuscles or lymph-cells. A pair of small arteries and 
veins enter the villus, forming a capillary network, while fine 
unstriped muscles in its walls contract it and force the chyle into 
the lymphatic vessels. In the figure, on one side of the villus is 
shewn a Lieberkiihn’s gland, since such are generally associated 
with the villi. 
Fic. 1.—D1aGRAM OF AN INTESTINAL VILLUS WITH THE CENTRAL ABSORBENT, RAMIFIED CANAL. 
L.v. its duct ; Aand v, Artery and vein ascending in Sm. the submucous layer ; Z. Cylindrical 
cells of the epithelium of the mucous layer, which at L.G. forms a Lieberkiihn gland; Lg. and 
An. Longitudinal and annular or circular muscular fibres ; Se. Serosa or outer layer of connective 
tissue, together with the investing peritoneal lamella Pe., which forms the mesentery M. in Fig. 2. 
L Follicle at base of villus 
Fic. 2.—DIAGRAM OF A TRANSVERSE SECTION THROUGH THE INTESTINE. 
V. Villi; M. Mesentery with blood- and lymphatic vessels. 
The capacity of the Intestinal Canal depends upon the nature 
of the bird’s food. In order to compare its length in different 
forms we require a unit by which to arrive at its relative propor- 
tions. The length of the whole vertebral column, or even the dis- 
tance from the tip of the bill to that of the tail, has been 
frequently used; but this gives only faulty results, since the 
length of the neck is obviously not correlated with that of the intes- 
tine. Numerous measurements and comparisons have led me to 
adopt as the unit the distance from the first thoracic vertebra (7.e. 
from the root of the neck) to the anus, and thus the quotient of 
the absolute length of the intestine from the pylorus to the anus is 
the relative length of the gut. This relative length is very con- 
stant in a species, and often gives results of considerable taxo- 
nomic value. Of course “short-gutted” and “long-gutted” are 
