DIGESTION 445 



fat goes into the lymphatic vessels of the villi and so to the general 

 circulation (p. 453) and the fat depots of the body. Water is 

 largely absorbed in the colon. 



The enzyme system of vertebrates is not capable of breaking 

 down the cellulose cell walls of plants, which make a great part 

 of the food of the rabbit. In the caecum, where the food stays for 

 some time, they are attacked by bacteria, which may be regarded 

 as symbionts. They change the cellulose into a form which the 

 rabbit could use, but it is below the point where digestion and 

 absorption are possible ; there has therefore been developed a 

 peculiar type of feeding called refection, or pseudorumination. 

 Freshly eaten food passes very quickly through the stomach 

 and small intestine to the caecum. Early next morning it is passed 

 out as soft faeces without drying, and these are at once eaten by 

 the rabbit. They pass to the cardiac stomach and remain there 

 while fresh food passes straight through as before. The faeces-food 

 is digested in the ordinary way in the stomach and intestine ; it 

 does not re-enter the caecum, but passes slowly through the large 

 intestine so that water is absorbed and the normal dry faecal 

 pellets are produced. Refection is specially important in giving 

 an adequate supply of protein, and perhaps of vitamin B. It 

 increases in cold weather. 



DUCTLESS GLANDS 



The ductless glands, which secrete the hormones (p. 18) are 

 conveniently listed here, although they do not form a single 

 anatomical system. The adrenals are a pair of yellowish kidney- 

 shaped bodies close to the points where the renal arteries leave 

 the dorsal aorta ; the right is a little anterior to the left. The 

 pituitary is situated" in the sella turcica (p. 427) ; it has lost its 

 connection with the roof of the pharynx (p. 368) but retains 

 that to the brain. 



The thymus is a soft, pink mass in the mediastinal space at 

 the front of the thorax. The thyroid is a thin, red body consisting 

 of two lobes, one at each side of the larynx, joined by a band 

 across the ventral side of the latter. Other ductless glands associ- 

 ated topographically with exocrine organs are the islets of 

 Langerhans in the pancreas, and the interstitial cells of the 

 gonads. 



