COURSE OF THE INGESTA. 95 



foecal odour; yet additional matters continue to be absorbed 

 from them in their passage onwards. In. the colon both the 

 longitudinal and circular muscular fibres of the walls are 

 collected into bundles; the longitudinal fibres* forming three 

 bands, and the circular dividing the intermediate spaces into 

 shallow recesses; and thus, although the total diameter of 

 this part of the intestine may exceed two inches, its contents 

 are brought well into contact with its walls as they lie in 

 these recesses ; and as they are pressed onwards from one 

 recess to another, different portions are brought to the sur- 

 face. The rectum, or lower bowel, has the muscular fibres 

 regularly disposed around, except near the outlet, where the 

 circular fibres form a strong sphincter, or habitually con- 

 tracted ring of muscle, on which the power of retention is 

 principally dependent, although it "is assisted by striped 

 muscles when the pressure on it is great. 



64. The form of digestive tube, which has been briefly 

 described, is that which is found in the human subject, but 

 there is great variety in different animals. In certain fishes, 

 the pipe fishes and others, the tube is straight, but in most 

 of them it is convoluted ; and in all it is wide from the 

 throat to the pylorus, then constricted, and again widened 

 towards the termination; this latter enlargement being 

 evidently the representative of the large intestine. A valve 

 even exists at the entrance into the large intestine in many 

 fishes. Thus it will be seen that already in the lowest divi- 

 sion of vertebrate animals, the digestive tube is divisible into 

 three great parts. In mammals neither the part above the 

 pylorus nor that below the ileo-colic valve has a uniform 

 breadth ; the stomach is separated from the pharynx by a 

 narrow oesophagus; and the caecum is in some instances 

 greatly larger than the rest of the great intestine. The 

 stomach and caecum are the parts which undergo the most 

 remarkable complications. The human stomach is an instance 

 of the simplest and typical form of the organ in mammals ; 

 but in various animals belonging to widely separate orders, 

 remarkable complications exist, e.g., in the camel, the kan- 

 garoo, the cetacea, the peccary, and certain monkeys. 



Probably the arrangement most interesting to the general 

 reader is the ordinary ruminant stomach^ which is clividecl 



