The Nervous System. 97 



roots, one motor ( the lower) the other sensitive, (upper) 

 they unite and later on are combined as a mixed nerve, 

 distributing their upper fibres to the spinal muscles and 

 skin, the lower to the sides and lower parts of the trunk 

 and limbs; fibres also being sent to join the other 

 (b-or sympathetic) system. The nerves of the limbs are 

 derived from large plexuses in the armpit and groin, 

 and are generally deep-seated, accompanying the arteries 

 in their course. B. The Sympathetic system is found 

 underneath the spinal column as two chains, with en- 

 largements along their course, as has been stated; they 

 are the basis of nerve supply to the organs of nutrition, 

 such as the lungs, liver, heart, intestines, blood vessels, 

 etc. Surrounding the brain and spinal cord are three 

 membranes (duramater, arachnoid and piamater) be- 

 tween them a fluid is found, the purpose of these is to 

 protect such sensitive and important structures (the brain 

 and cord)' from concussion and injury. The proportion- 

 ate size of the brain and spinal cord varies in different 

 animals according to the place they occupy in the scale of 

 intelligence; thus the greater the weight of the brain the 

 lighter the spinal cord, c g., in Man the brain weighs 

 fifty ounces, the cord one and one-half ounces; in the 

 horse the brain weighs twenty-three ounces, the cord 

 ten and one-half ounces; the proportions respectively 

 being 33 to 1 and 2.2 to 1. The convolutions of the 

 cerebrum are separated by depressions (sulci), the 

 deeper these are the higher the intelligence. Captain 

 Hayes rates the intelligence of animals in the follow- 

 ing descending scale: Man, dog, cat, ass, pig, horse, 

 ox. The nervous system of animals therefore may be 



