486 Intestinal Folds and Villi in Vertebrates 



A typical form of villus is found in the flesh-eating animals. The 

 villi are rather long, thick, cylindrical or finger-shaped, such as found 

 in cat, dog, lion, etc. With insectivorous mammals, this form is slightly 

 modified; the villi may have sharper tips. The villi of insectivorous 

 birds are similar to those of corresponding mammals, but they differ 

 from them in being more flattened. With Kaptores, the villi, although 

 more or less flattened, approach the cylindrical or columnar type. 



In the vegetable-eating mammals, and some birds past the fold stage, 

 three general types of villi may be recognized: 



1. The leaf -like or thin, broad, tongue-shaped form. 



2. The long, slender, cylindrical or thread-like form. 



3. The short, thick, columnar or wart-like form. 



The first sort is the most common and has already been described 

 with some modifications in rabbit, mouse, muskrat and monkey. Also 

 in some birds. 



The second or thread-like sort of villus is found in the ileum of some 

 rodents, such as rabbit. This type is found, for example, in the intes- 

 tines of cow, camel and sea-cow. 



The third or thick, columnar to low wart-like forms are found in the 

 cseca and large intestines of some grain-eating birds, as chicken, turkey, 

 partridge, and is also found in the small intestine of horse. These 

 villi differ from the carnivorous type because of their proportionately 

 broader bases, less height, and fewer numbers. 



With omnivora, as a usual thing, villi in mammals correspond with 

 either the carnivorous or the herbivorous type; for instance, tlie rac- 

 coon has the carnivorous type and the opossum has the herbivorous type 

 of villus. Seldon do omnivora combine both types in the same animal; 

 however, in man such a condition may be said to exist; that is, there 

 are leaf-like villi in the upper intestine or duodenum and columnar in 

 the ileum. 



The development of the first villi in chick very interestingly shows 

 a number of the^stages of folds which were found in the adult intestines 

 of lower forms, as well as the conditions existing in a number of adult 

 birds. The much later development of villi in the cffica and large intes- 

 tine does not show all the fold stages which were found in the small 

 intestine. If these stages were ever developed, there is very little indi- 

 cation of them at present. 



The first development of villi in mammals is from straight, parallel 

 folds, no zigzag folds are formed like those in chick. It may be that 

 the zigzag folds were never developed, but it seems at least possible that 

 this zigzag fold stage was once formed, but now entirely omitted, be- 



