,70 THE RABBIT. PHYLUM CHORDATA 



sense organs of taste, known as taste-buds, are found in various 

 parts of the mouth, principally on certain elaborate papillae of 

 several kinds on the tongue, and are supplied with fibres which 

 run in the glossopharyngeal nerve and in the mandibular branch 

 of \'ll. The nature of the sense of taste in the rabbit, though per- 

 haps not in lower animals, may be inferred from that of man. In 

 man the only true ' tastes ' are the perceptions of the qualities 

 sweet, sour, bitter, salt, and perhaps metallic and alkaline. 

 Other so-called tastes are aromas perceived by the organ of smell, 

 to which traces of the substances that give rise to them are 

 conveyed by air from the mouth through the posterior nares. The 

 rabbit has also sense cells in the skin which are sensitive to touch 

 and temperature, and in the muscles which respond to their 

 tension and so enable the animal to maintain its tone and its 

 posture and gait. 



HORMONES 



Another method of control and co-ordination, which is found 

 in all vertebrates and in some of the more advanced inverte- 

 brates such as arthropods and molluscs, is by hormones or chemical 

 messengers. These are substances which are produced in the body 

 and released into the tissues (typically into the blood stream) 

 whence they are carried or diffuse to one or more other parts 

 of the body, on which they have either a specific or a general 

 effect. The chief producers of hormones in the rabbit are the 

 ductless glands which we have listed on p. 445. As an example 

 of a hormone of general effect we may take that produced by 

 the thyroid ; its exact chemical nature is not certainly known, 

 but it is related to the substance thyroxine which has been ex- 

 tracted from the gland, and is probably this substance united 

 to a protein. It is continually being formed, and is necessary to 

 maintain the activity of the body ; it seems to determine the 

 basal metabolic rate (i.e. the rate at which oxygen is used in 

 the absence of any special activity) and although its secretion 

 probably varies little, the increased metabolism of fevers and on 

 exposure to low temperatures may be caused by excess of it. 

 The adrenaline produced by the medulla of the adrenal gland 

 is another general hormone, but its activity is most marked 

 when some special stimulus — low temperature, lack of oxygen, 

 or (in man) the emotional states of fear or anger — causes its 



