463 Intestinal Folds and Villi in Vertebrates 



the spiral valve is present;, the intestine, although of considerable 

 diameter, is very short. Hence, a very important difference, physiolog- 

 ically, may be noticed between the spiral valve and valvulse conniventes, 

 the first developed especially for retention or retardation of the food 

 and greater compactness, and the second, formed especially for increased 

 absorption area. The important difference already hinted at, is that 

 spiral valves were formed from an infolding to a greater or less extent 

 of the whole muscular walls as well as of the mucosa and submucosa, 

 while valvulsB conniventes were formed by a simple fold of the mucosa 

 and submucosa. This theory is partially based upon the following 

 facts. 



In the spiral valve of Lepidosteus and Amia, there is more muscular 

 tissue than could have been derived from a double layer of the mus- 

 cularis mucosas. 



There is a spiral fold in the caecum of rabbits. When rabbits are 

 born, this structure is not fully developed, its presence is marked on 

 the exterior by constrictions, sections across these constrictions show 

 the developing folds on the ental surface and the muscle of the mus- 

 cular coats can be traced for some distance into the spiral core (Fig. 41). 

 Of course this fold in the rabbit may not be homologous with the spiral 

 valve of other forms, and the hypothesis outlined above requires much 

 further investigation before it can be substantiated. 



It seems that man is the only animal who can justly lay claim to 

 valvulce conniventes. It might naturally be supposed that some of the • 

 anthropoid apes would possess structures corresponding to some stage 

 of these folds, however, careful examinations of specimens and a review 

 of the literature on apes did not bring to light any clue to such 

 structures. 



The valvulce conniventes of man begin their development shortly 

 before birth and do not reach their full height until long after the 

 villi are formed. They begin as small, transverse, semi-ringlike thick- 

 enings in the submucosa, these grow more prominent, those below the 

 pylorus some distance become prominent more rapidly than others; as 

 these thickenings project inward, they carry on their surfaces the villi' 

 of the mucosa (Fig. 42). The structure and arrangement of valvulse 

 conniventes in the adult are so well known as to require no redescrip- 

 tion. 



One particular more than another which may be worthy of note is 

 the following: Brooks, 92, and Kazzander, 92, described valvulge con- • 

 niventes and both recognized in some cases, a spiral arrangement of 

 the transverse foldings. Just what this signifies, it is difficult to 



