748 COMPARATIVE ANATOMY 



intestine is the caecum. It is the tapering end of this caecum which 

 forms the appendix (Fig. 436) in man and in some of the other verte- 

 brates. There is also a valve at the junction of the ileum and caecum 

 known as the ileo-caecal valve. 



The first part of the small intestine which follows the duodenum 

 is known as the jejunum, while the more distal portion is the ileum. 



Professor Wilder gives the following interesting account of the ap- 

 pendix and succeeding structures : 



"At the junction of the small intestine with the large, there is a 

 strong tendency to form one or more caeca, or blind sacs, which often 

 become digestive organs of great physiological efficiency. The charac- 

 teristic form in reptiles is that of a single rather short and wide caecum, 

 symmetrically placed. In birds there are usually two symmetrical ones, 

 which attain great length in scratching birds (e. g., the common fowl), 

 and in ducks and geese, but are quite rudimentary in certain others 

 (woodpeckers, parrots, etc.). Ostriches possess a single caecum of 

 great length (seven to eight meters) and furnished with an internal 

 spiral partition, which greatly increases its effective surface. 



"In mammals a single caecum is developed, which varies greatly 

 in size and functional importance. Rudimentary in edentates, most in- 

 sectivores, and bats, it frequently attains an enormous size in herbivorous 

 or graminivorous forms. In certain rodents (e. g., muskrat, woodchuck), 

 its total capacity equals- or exceeds that of the remainder of the alimen- 

 tary canal, and in the marsupial Phascolarctus it is three times the 

 length of the body. In the rabbit it is provided with an internal spiral 

 valve ; in certain other rodents and in the higher apes and man, the free 

 end becomes rudimentary, restricts its lumen, and forms a worm-like 

 process, the processus (appendix) vermiformis, which like all rudimen- 

 tary organs, is subject to a large amount of individual variation. 



"Thus in the human subject the appendix varies in length between 

 the limits of 2-23 cm., the average for an adult being 8-9 cm. It is longest 

 proportionally during fetal life, its length relative to that of the large 

 intestine being 1 :10, while in adult life it is 1 :20. It is longest abso- 

 lutely between the ages of ten and twenty, after which it shows a slight 

 reduction. Its status as a rudiment of slight functional value is shown 

 by the tendency toward the obliteration of its lumen, a tendency which 

 increases steadily with age. Furthermore, these two characters, reduc- 

 tion in length and obliteration of the lumen, go hand in hand, short ap- 

 pendices being usually solid, while large ones are apt to possess a lumen. 



"The position and arrangement of the colon varies considerably 

 among various mammals. In man it begins low down on the right side, 

 from which there proceed in order an ascending, transverse, and de- 

 scending portion, connected with the rectum by a sigmoid flexure, 

 through which the tube attains the median line ; a similar disposal is 

 seen in many other anthropoids, in lemurs and rodents, the majority of 



