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 is only once susceptible ; but it is observed to be liable to 

 the attack of these diseases, at every period of human life. 



URINARY SYSTEM. 

 t 



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 excrementitidus 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 peritonaeum, 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 into the great cavity of the kidney, termed its 

 pelvis. From this pelvis or receptacle, a tubular irregular 

 vessel, termed the ureter, conveys the urine to the bladder. 

 The kidneys are supplied with blood from the aorta. The 

 veins return it to the 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 



