Wniiam A. Hilton 467 



(b) With villi which are not regularly arranged. This condition 

 exists in the intestine of ruffled grouse, or Bonasa umbelltis, domestic 

 turkey, chicken, humming bird {Trochilus colubris), red-eyed vireo 

 (Vireo olivaceus) and many others. It has been described by Gadow 

 for a number of forms including the genus Corvus of Passeres. 



In the following table (page 4:68) particular attention is paid to the 

 form, size and number of villi in several species of birds which were 

 carefully studied. 



It may be seen from the preceding descriptions and the table on next 

 page that there are really only two distinct forms of villi with many 

 slight variations. These are the thin leaf-like form and the tooth- 

 shaped columnar form. It may also be observed that in birds villi or 

 folds are found to a greater or less extent in the large intestine and 

 caeca as well as in the small intestine. 



The occurrence of folds in the intestines of adult mammals seems 

 to be exceptional and villi are in most cases entirely confined to the 

 small intestine.* 



Cuvier, lo, Meckel, 26, Owen, 47, Leydig, 47, and others, have de- 

 scribed ring-like somewhat spiral or oblique folds for Ornithorh5nichus ; 

 these are most numerous near the pylorus and extend into the first 

 half of the colon, leaving the caudal half smooth. Small " secondary " 

 projections were described on these folds by Oppel, 97, but he was 

 unable to state definitely the homology of either of these structures. 



Folds have also been described in some of the Edentata, Cetacea and 

 Sirenia, by Meckel, 19, Eapp, 37 and 43, Eschricht, 49, and several 

 others; Leydig, 57, Forbes, 79, have described large folds in elephants. 



Villi are usually described upon the folds in elephants. In a number 

 of species of mammals where folds have been described no reference is 

 made to the occurrence of villi. Unfortunately I was unable to ex- 

 amine any specimens of the mammals in which these folds have been 

 described. 



The forms of villi in mammals may be grouped under four heads: 



1. Thin, leaf -like or plate-like villi. 



(a) With square corners, as found in the intestine of musk-rat (Figs. 

 14-16). 



(b) With rounded edges, as in the duodenum of man, the intestine 

 of apes and monkeys and a number of rodents (Figs. 31-23, 25-27, and 

 34, 35, 37). 



3 Little mound-like elevations are present in the Cfecum of rabbit. These are 

 found to have central lacteal vessels and a capillary networks of blood vessels; and 

 so may be spoken of as villi although somewhat reduced in size. 



