218 DIGESTION. [CHAP. xxv. 



the small intestine to the spine is the mesentery } and each portion of 

 the large intestine is connected to the corresponding region of the 

 abdominal wall by a process of peritoneum, which is designated by 

 prefixing the word meso (//-ecro? medius) to the name of the parti- 

 cular portion of intestine : thus mesoccecum is the process which con- 

 nects the coecum to the iliac fossa; mesocolon, right, transverse, left, 

 is that which belongs respectively to the three portions of the 

 colon; and the mesorectum connects the rectum to the concavity of 

 the sacrum. 



Attached to the colon are small processes of peritoneum, con- 

 taining fat, and called appendices epiploicce, from their resemblance 

 to the epiploon, or great omentum, which descends from the great 

 curvature of the stomach and from the transverse colon, like a 

 curtain, in front of the small intestine. 



The Intestinal Canal in Vertebrata. The intestinal canal is disposed in the 

 four vertebrate classes much on the same plan as in the human subject: 



In Fishes, the intestinal canal exhibits for the most part a very simple con- 

 formation. In many fishes, it passes quite straight, or very nearly so, through 

 the body : when it does exhibit convolutions, they are few and short, and rarely 

 to any great extent. A pyloric valve is generally present, separating the intes- 

 tine from the stomach; immediately succeeding to this valve, the intestine 

 generally experiences a dilatation, whence it gradually contracts to its terminal 

 portion, which again becomes dilated. This portion corresponds to the large 

 intestine, and commonly a valvular fold of the mucous membrane is present 

 at its union with the preceding portion ; it terminates in a cloaca common to it 

 with the genital and urinary organs. In some fishes, however, no dilatation is 

 found, nor any external distinction between the stomach and intestine, and the 

 canal from the oesophagus to the anus is of uniform calibre. Immediately below 

 the pylorus, we very commonly find a series of tubular prolongations from the 

 intestine, terminating in blind extremities. These constitute the appendices py- 

 loric<B,or pyloric follicles which most comparative anatomists consider to supply 

 the place of a parenchymatous pancreas. These appendices vary considerably 

 in both number and size. In Pleuronectes fiesiis there are only two very short 

 ones, placed opposite each other at different sides of the intestine ;* in Ammody- 

 tes tolianus there is only one ; in Blennius and Gasterosteus there are only two, so 

 small that Wagner compares them to the follicles of the proventriculus of birds ; 

 there are from ten to thirty in many species of Clupea and Salmo, and in the ge- 

 nera Gadus and Scomber (the common mackerel, for example), there are as many 

 as two huudred.f On the other hand, they are entirely absent in many fishes. 

 Again, in some they are simple, in others they become subdivided or branched 

 at their blind extremities, and in others still these branches undergo subdivision, 

 and the resemblance to the glandular formation is enhanced by the fact, that 

 these branchings are connected by means of cellular membrane and blood-vessels. 



In Reptiles, the intestinal canal differs from that in fishes, chiefly in having 



* Figured in Carus's Anat. Comp. pi. ix. fig. 20. 

 t E. Wagner, Vergleichend. Anatom. 



