140 Zoological Society. 
portion of its aorta, which I shall briefly describe with the hope of 
leading to further inquiry. Before the ischiatic arteries are given off, 
the aorta is comparatively large and is enveloped externally in a dense 
fibrous coat, possessing very little elasticity : below the origin of these 
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- 
lieving that that coat is not truly muscular, but rather of the nature 
of the muscular coat of the intestmes, 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 
Chimera 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 vertebree,—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 mtestine 
is almost as amply provided with villi as its small; and that even the 
isthmus or narrow neck of each of its large czeca is similarly provided 
with 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 found them. 
29. Norres oN THE SKULL oF Eauus Hemionus anp Equus 
Kianc. 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 
