29 



arteries, the trunk of the aorta suddenly becomes small, and continues 

 small and tapering to its termination ; and this change is accompanied 

 with an alteration in the structure of its external coat. In place of a 

 dense fibrous envelope, it is now sheathed in a substance very like 

 muscular fibre, and which from its properties I believe to be a mus- 

 cular layer. It is of some thickness, of a reddish hue, slightly elastic, 

 easily broken, and divided by a ligature and easily separated into 

 longitudinal fibres of considerable length. Under the microscope 

 each filament appears to be composed of nearly parallel fibres of ex- 

 treme delicacy, and destitute of those peculiar markings which be- 

 long to the fibres of the voluntary muscles generally and to some of 

 the involuntary. Moreover, when placed in a warm damp atmo- 

 sphere, at a temperature between 80° and 90° Fahr., it rapidly putre- 

 fies and is reduced to a poultaceous or semifluid consistence. These 

 properties seem to characterize it as a muscular structure ; I would 

 not dwell on any one in particular, but rather on the assemblage of 

 them. An attempt of late has been made to revive the old doctrine 

 of the muscularity of the middle coat of the arteries, founded almost 

 exclusively on microscopical appearances. The structure described 

 above, I consider not of the nature of the middle arterial coat, be- 

 lie^dng that that coat is not trulji^ muscular, but rather of the nature 

 of the muscular coat of the intestines, to which, in point of colour, 

 consistence, the effect of a ligature, its microscopical appearance and 

 proneness to putrefy, it is so very similar. 



If this structure be admitted to be muscular, it may be viewed as 

 accessory and of a use similar to that of the accessory hearts of the 

 Chimsera and Torpedo, and destined to some peculiarity of function 

 which further research is required to determine. 



Before concluding this notice, I may mention incidentally that I 

 availed myself of the opportunity afforded by this Swan to examine 

 the air contained in its osseous air-cells. I found it to be composed 

 of about 83"3 per cent, azote, and of 16-7 per cent, oxygen, tested by 

 means of lime-water and phosphorus. It was collected from the cells 

 belonging to the cervical vertebrae, — cells by means of which this part 

 of the bird is happily buoyant, floating in water, even when deprived 

 of its feathers and integuments and detached from the trachea. And, 

 further, I may mention, which was new to me, that its large intestuie 

 is almost as amply provided with villi as its small ; and that even the 

 isthmus or narrow neck of each of its large caeca is similarly provided 

 Math villi. Some other animals, especially birds, may be analogous 

 in this respect ; but in no other instance in which I have yet examined 

 the large intestines in search of villi have I foimd them. 



2. Notes on the Skull of Eauus Hemionvs and Eauus 

 KiANG. By J. E. Gray, Esa., F.R.S. 



Mr. Hodgson has lately sent to the British Museum three speci- 

 mens of the Horse, which he had described under the name of Equus 

 Kiang ; unfortunately they were so destroyed by insects during their 

 passage from India, that it was impossible to preserve any part of 

 them except the skull and the bones of the limbs. 



