18/2.] (ESOPHAGUS OF THE PIED HORNBILL. 1" 



Hence it seems interesting to examine the oesophagus of the birds 

 alluded to above ; and I have in former memoirs shown that the 

 Owls and Hawks, like other birds, are devoid of an oesophageal sheath 

 of transversely striated muscular fibre. But the remarkable power 

 possessed by the Hornbills, of ejecting matters through the oesophagus 

 upwards, suggests the necessity of extending the examination to these 

 birds especially. And this I have done, so far as regards the species 

 mentioned at the head of this paper. 



The sheath of the oesophagus was thin, of a reddish colour, and 

 composed of fibres almost if not completely arranged transversely, as 

 is usual in birds. These fibres appeared in bands or fascicles, each 

 about ^lyth of an inch thick, and made up of smooth fibrils closely 

 connected together, each about -4-^rjyth of an inch in diameter, and 

 the whole of them so thickly studded with nuclei as to make the 

 fibres appear speckled when examined under an achromatic object- 

 glass of one tenth of an inch focal length. Not even a single trans- 

 versely striated fibre or fibril could be found on the oesophagus lower 

 than the termination of the pharyngeal muscles. 



In short, there was no transversely striated muscular fibre on the 

 true oesophagus ; and thus this bird affords no exception to the rule 

 of a want of this kind of muscle on the oesophagus of the class. 

 And hence, so far as is at present known, Mammals and Fishes 

 might be truly defined as Vertebrates with transversely striated 

 muscular fibre on the oesophagus, and Birds and Reptiles as Verte- 

 brates devoid of a transversely striated oesophageal muscular sheath. 

 How the differences in the extent of this fibre along the oesophagus 

 afford good taxonomic characters in the Mammalian class has been 

 explained in my papers published in the ' Proceedings' of the Zoolo- 

 gical Society, June 14, 1842, April 22, 1869, and May 12, 1870. 



The colour of the muscles of animals is often, but by no means 

 always, correlated with their energetic action. Neither is' the colour 

 regularly indicative of the intensity of the transverse markings in 

 animals generally, nor is it redder in the oesophagus of Ruminants 

 particularly than in many other Mammals. In Insects of the most 

 active habits, and in numerous energetic Fishes and Lizards, the 

 muscles are pale, and yet more forcibly marked with transverse stripes 

 than the deeply coloured muscles of many Birds and Mammals, as is 

 well known of the Skate and Man to teachers of histology. Leydig, 

 indeed, maintains that this distinctness or largeness of the striae is 

 related to the activity of the muscles, and adduces Insects as exempli- 

 fications of his opinion. But the muscles of many sluggish Cater- 

 pillars and of some other equally tardy Arthropoda are quite as 

 plainly striated. In several birds, as the common Swift (Cypselus 

 apus), the transverse striae of the fibres of the wonderfully active 

 pectoral muscles are much less distinct than in the comparatively 

 idle muscles of the legs (Proc. Zool. Soc. June 14, 1842). Though 

 in Ruminants the transversely striated muscle of the oesophageal 

 sheath extends to the stomach, there is a large proportion of smooth 

 muscular fibre on the last inch or two of the oesophagus. And in 

 many other Mammalia, as, e. y., certain Bears and Rodents, the 

 Proc. Zool. Soc— 1872, No. II. 



