NEKVODS SYSTEM. 315 



4. rrom materials derived from the tissues — products of 

 decomposition, and solution of portions of these tissues due to 

 their vital activity (lymph). 



5. From a small amount of matter that may be absorbed by 

 the skin. 



The blood gives up its A^aste nitrogenous material in the 

 kidneys ; water is also lost l)y its progress tln'ougii the same 

 organs, as well as vario\is salts. Tlie skin also yields water and 

 salts derived from the ^-ascular fluid. The COo and aqueous 

 vapour are passed out in the pulmonary sacs, whilst the oxygen 

 is absorbed by the blood in the same area. Blood also loses 

 material during its passage through the organs, &c., for their 

 constructive purposes. This waste is counteracted by the five 

 methods named above, from which the blood is built up and its 

 losses repaired. 



The iiei'vous system is divided into two distinct portions — 

 the most important being the " central " nervous system, the 

 spinal cord and brain. The other is the "sympathetic '' nervous 

 system. The organs of the liody are under the control of the 

 nervous system. Muscle and nerve are intimately connected ; 

 each muscular act is preceded by a nervous act : sometimes 

 these acts are voluntary, at others involuntary. The move- 

 ments of the muscles of the limbs, &c., are voluntary acts ; the 

 muscular movements of the heart and intestines are involuntary, 

 and yet both are caused by nervous mechanism. 



The brain and spinal cord form the central nervous or 

 cerebro-spinal axis. The brain is deposited in the cavity of 

 the skull, the cranium; the spinal cord is protected by the 

 bony vertebrae, and lies in the neural canal. The cerebro-spinal 

 axis is surrounded by three membranes — the dura-mater, the 

 arachnoid, and the picc-mater. The dura-mater forms the ex- 

 ternal layer next the lione of the skull ; the arachnoid is a 

 serous layer ; whilst the pia-mater closely invests the brain and 

 nervous axis, forming a nutrient membrane. The brain varies 

 much in weight in different animals : in the horse its normal 



