46 STRUCTURE AND CLASSIFICATION OF BIRDS 



are structures in birds comparable to the diaphragm of 

 mammals; but in that case the likeness would be due to the 

 derivation of both from a common form, perhaps low down 

 in the reptilian series. At any rate certain anatomical facts 

 forbid a precise comparison of the entire oblique septa of the 

 bird to the mammalian diaphragm, though, as will be shown 

 in the sequel, there appear to be a few genuine resemblances 

 which can hardly be got over. HUXLEY emphasised the fact 

 that in the bird the heart lies behind the so-called diaphragm, 

 which is moreover not supplied by a phrenic nerve. If com- 

 parable to any structure in the mammal, it is with the medias- 

 tinum that they should be homologised. As to the phrenic 

 nerve, several considerations present themselves; it is, it is 

 true, a specialised nerve, but it is spinal in origin. Now in birds 

 it is also branches of spinal nerves which supply the oblique 

 septa. In the mammal it may be that the pulling out of the 

 phrenic nerve may be due to a cause similar to that which 

 has produced the looping of the recurrent laryngeal nerve, 

 When nerves are drawn out by a change in position, or by an 

 elongation of the structures which they supply, there is at 

 least a tendency for their roots of origin, of many, to fuse 

 into a single nerve ; witness, for instance, the limb nerves 

 arising from the anterior and posterior plexuses. The fact 

 that the spinal nerves which form the limb plexuses are not 

 always exactly the same has not led to any very serious 

 belief in the serial homology only of the fore limbs in any 

 two vertebrates which show these differences. The lungs, 

 and consequently the diaphragm (assuming for a moment its 

 correspondence with the mammalian diaphragm), are further 

 back in birds ; hence their different nerve supply. If we 

 look upon the posterior portion of the oblique septa, which is 

 alone, be it observed, muscular, as the homologue of the 

 lateral parts of the mammalian diaphragm, the rest being 

 absent, no great violence to the mutual relations of the 

 different structures concerned will have been done. In any 

 case it may well be that both the mammalian diaphragm 

 and the avian have been derived from some such reptilian 

 structure as is to be seen in the crocodile. 



