DISEASES OF THE BLOOD VESSELS 67 
The vessels of mammalia retain a considerable wall 
throughout nearly their entire length. At first the wall 
is thin compared to the calibre of the vessel while the 
arteries smaller in calibre, have a heavy wall. In birds 
the arterial stalk at the heart is supplied with very heavy 
walls, but after the second branching the relation of wall 
to calibre seems to continue about the same. In this class 
the stalk vessels have wall to calibre relation of 1 to 3 
(measurements in 2 Passeres, 1 Psittaci, 1 Accipitres) 
whereas in mammals the relation varies from 1 to 5 to 1 
to 7 (observations on 2 carnivores, 1 ungulate, 2 rodents). 
In mammals the consistency of a normal artery wall 
remains much the same, a firm, resilient, yellow-white 
tissue, quite opaque and standing open upon cross 
section. In birds this description covers the main stalk, 
the aorta in the abdomen and the first part of the carotid 
and iliacs. When these characters are lost, the arteries 
become semitranslucent bluish strands so that they are 
difficult to follow in the muscles of the neck and extremi- 
ties. This is particularly true in the Passeres, Picarige, 
Galli and Columbse while in the Psittaci, Accipitres, and 
Anseres the arteries are distinctly whiter than the veins 
but yet quite soft. In the Herodiones and Struthiones, 
thick walled vessels may be followed as far as the second 
joint in both extremities. These differences depend in 
part upon the grosser construction of the central arteries 
in Aves and in part upon the larger amount of elastic 
tissue in them than in the secondaries and smaller vessels, 
and than in comparable mammalian vessels. 
It is impracticable to go into the minutiae of histology 
in the different orders, which indeed varies but little, 
although attention might be directed to the facts that in 
all central vessels the relative amount of elastica is 
greater than in smaller ones and that muscular tissue 
seems to exceed in the latter. Considerable work has been 
done upon the amount and arrangement of muscle bands 
in isolated genera, but no comprehensive data are at hand 
