-«2 THE RABBIT. PHYLUM CHORDATA 



There is thus a series of processes which is largely self-regula- 

 tory. If, when a cycle is finished, the external conditions are still 

 right, another may follow, and the animal is said to be poly- 

 oestrous, but in the larger mammals, with a long gestation period, 

 the oestrous cycle fits just once into the seasonal cycle. A similar, 

 but less clear, cycle occurs in the male. 



It is clear that hormones are less well adapted to precise 

 connections than the nervous system, and in fact most of them 

 affect structures in more than one part of the body. It is necessary 

 that hormones should be readily diffusible and easily destroyed, 

 or they would be too slow in action and would persist after their 

 effects were no longer required. In fact they are all of relatively 

 low molecular weight and are rapidly destroyed in the body. 

 Thyroxine is the only one which is not digested in the gut. It 

 is probable that they take part in chemical reactions in the body, 

 either as enzymes or as coenzymes. It is in fact difficult to frame 

 a definition which will separate them from enzymes in general, 

 and they differ from vitamins only in that they are made in the 

 body, while vitamins must be taken in with the food ; what is 

 a vitamin for one species may be a hormone for another. 



In view of the fact that part of the action of the nervous 

 system depends on the liberation of chemical substances, some- 

 times called neurohumours, it is tempting to think that a single 

 primitive co-ordinating system has evolved along two lines, one 

 ending in the vertebrate endocrine system, the other in the verte- 

 brate brain. The adrenal medulla, which is formed from cells of 

 nervous type, remains as a link, for its secretion adrenaline is 

 similar to, and possibly identical with, the sympathin liberated 

 by the ends of sympathetic nerve fibres ; as we have seen, the 

 two substances have the same effects. 



THE RAT 



The laboratory rat, a cage-bred variety of the brown or Norway 

 rat, Rattus norvegicus, is often dissected as a mammalian type 

 instead of the rabbit. It is now considered to belong in an order, 

 the Rodentia, distinct from that called Lagomorpha in which the 

 rabbit is placed, but the two species do not differ fundamentally. 

 The chief points of difference which the elementary student might 

 notice in dissection are listed below ; there are also some differ- 

 ences in the skeleton, which is seldom studied. 



