DIGESTIVE SYSTEM 



139 



as well. The structure of these small hut 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, Avhich 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 miiscles 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. 



Fig. 1. Fig. 2. 



Fig. 1. — Diagram of an Intestinal Villus with the Central Absorbent, Ramified Canal. 

 L.v. its duct ; A and v. Artery and vein ascending in 5m. the submucous layer ; Fj. Cylindrical 

 cells of til e epitlieliuiii of tlie mucous layer, which at L.G. forms a Lieberkuhn gland; Lg. and 

 An. Longitudinal ami annular or circular muscular fibres ; ,s'e. Serosa or outer layer of connective A. ., 

 tissue, together with the investing peritoneal lamella Pc, which forms the mesentery J/, in Fig. 2. jir^-r-<^ 



Fig. 2. — Diagram of a Transverse Section through the Intestine. 

 V. Villi ; jl/. Mesentery with blood- and lymphatic vessels. 



C0 



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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 Avith that of the intes- 

 tine. Numerous measurements and comparisons have led me to 

 adopt as the unit the distance from the first thoracic vertebra {i.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 



