380 HORSE, SWINE AND POULTRY DISEASES 



in symmetrical pairs, and are distributed to the skin, the voluntary 

 muscles, and the organs. In the second, the central organ consists 

 of a chain of ganglia connected by nerve cords which extends from 

 the head to the rump on each side of the spine. The nerves of this 

 system are distributed to the involuntary muscles, mucous mem- 

 brane, viscera, and blood vessels. The two systems have free inter- 

 communication, ganglia being at the junctions. 



Nerves are thrown into a state of excitement when stimulated, 

 and are, therefore, said to possess excitable or irritable properties. 

 The stimuli may be applied to, or may act upon, any part of the 

 nerve. Nerves may be paralyzed by continuous pressure 'being ap- 

 plied. When the nerves divide into branches, there is never any 

 splitting up of their ultimate fibers, nor yet is there ever any coales- 

 cing of them; they retain their individuality from their source to 

 their termination. 



Nerves which convey impressions to the centers are termed 

 sensory, or centripetal, and those which transmit stimulus from the 

 centers to organs of motion are termed motor, or centrifugal. The 

 function of the nervous system may, therefore, be defined in the 

 simplest terms, as follows: It is intended to associate the different 

 parts of the body in such a manner that stimulus applied to one 

 organ may excite or depress the activity of another. 



The brain is that portion of the cerebro-spinal axis within the 

 cranium, which may be divided into four parts the medulla oblon- 

 gata, the cerebellum, the pons Varolii, and the cerebrum and it 

 is covered by three membranes, called the meninges. The outer of 

 these membranes, the dura mater, is a thick, white, fibrous mem- 

 brane which lines the cavity of the cranium, forming the internal 

 periosteum of the bones ; it is continuous with the spinal cord to the 

 extremity of the canal. The second, the arachnoid, is a delicate 

 serous membrane, and loosely envelops the brain and spinal cord; it 

 forms two layers, leaving between them the arachnoid space which 

 contains the cerebro-spinal fluid, the use of which is to protect the 

 spinal cord and brain from pressure. The third, or inner, the pia 

 mater, is closely adherent to the entire surface of the brain, but is 

 much thinner and more vascular than when it reaches the spinal 

 cord, which it also envelops, and is continued to form the sheaths of 

 the spinal nerves. 



The medulla oblongata is the prolongation of the spinal cord, 

 extending to the pons Varolii. This portion of the brain is very 

 large in the horse ; it is pyramidal in shape, the narrowest part join- 

 ing the cord. The pons Varolii is the transverse projection on the 

 base of the brain, between the medulla oblongata and the peduncles 

 of the cerebrum. The cerebellum is lodged in the posterior part of 

 the cranial cavity, immediately above the medulla oblongata; it is 

 globular or elliptical in shape, the transverse diameter being great- 

 est. The body of the cerebellum is composed of gray matter ex- 

 ternally and of white matter in the center. The cerebellum has the 

 function of co-ordinating movements ; that is, of so associating them 

 as to cause them to accomplish a definite purpose. Injuries to the 



