CIRCULATING SYSTEM. 359 
years. This opinion, however, is rendered doubtful by many 
well known facts. Letters marked.on the skin by a variety 
of substances, frequently last for life. ‘There are some dis- 
eases, such as small-pox and measles, of which the constitu- 
tion 1s only once susceptible ; but it is observed to be liable to 
the attack of these diseases, at every period of human life. 
Urinary System. 
WE have been induced to add this section as an appen- 
‘dix to our observations on:circulation, from the persuasion, 
that the urine is an excrementitious fluid separated from 
the blood. The glands which are employed for this pur- 
pose, are termed kidneys. 
These organs, as they exist in quadrupeds, are two in 
number, one on each side of the spine, at the upper part of 
the loins. Each kidney is made up of numerous lobes, 
which are more or less intimately united according to the 
‘species. They are situated behind the peritonzum, and 
surrounded by a peculiar vascular membrane. ‘They con- 
sist of two parts, an exterior, termed cortex, .and an inte- 
rior medulla. In the cortical part, the urine is secreted, 
it then enters into conical shaped bodies in the medullary 
part. "These cones have their ‘base towards the surface 
of the kidney, and their apex towards the concavity of its 
central side. ‘They consist of hollow tubes, «which convey 
the urine intethe great cavity of the ‘kidney, termed its 
pelvis. From this pelvis or receptacle, a tubular irregular 
vessel, termed the wreter, conveys the urine to the bladder. 
The kidneys are supplied with blood from the aorta. The 
veins return it tothe vena cava inferior. ‘Their nervous 
energy is derived from the great sympathetic nerve. 
The bladder consists principally of two coats. ‘The ex- 
ternal is muscular, and consists of fibres which are various- 
ly decussated. The internal is a serous membrane, which 
