DISEASES OF THE HOESE. 191 



consists of a bundle of tubular fibers, held together b}' a dense areolar 

 tissue, and inclosed in a membranous sheath — the neurilemma. Nerv^e 

 fibers possess no elasticitj^ but are very strong. Divided nerves do 

 not retract. 



Nerves are throvv'n 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 nen^e. 

 Nerves may be paralyzed b}^ continuous pressure being applied. When 

 the nerves divide into branches, there is never any splitting up of their 

 ultimate fibers, nor yet is there ever any coalescing of them; they 

 retain their individuality from their source to their termination. 



Nerves which convey impressions to the centers are termed sensory^ 

 or centrlj^eial, and those which transmit stimulus from the centers to 

 organs of motion are termed motor, or Gentrlfugal. 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 ma}- excite or 

 depress the activity of another. 



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

 nium, which may be divided into four parts — the medulla oblongata, 

 the cerebellian, the pons Varolii, and the cerehruni — and it is covered 

 by three membranes, called the meninges. The outer of these mem- 

 branes, the dura mater, is a thick, white, fibrous membrane 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 araclinold, is a delicate serous membrane, and 

 loosely envelops the brain and spinal cord; it forms two layers, leav- 

 ing between them the arachnoid space which contains the cerebro- 

 sj^inal fluid, the use of which is to protect the spinal cord and brain 

 from pressure. The third, or inner, the^^/« mater, is closely adherent 

 to the entire surface of the brain, but is much thinner and more vas- 

 cular than vv^hen it reaches the spinal cord, v>^hich it also envelops, and 

 is continued to form the sheaths of the spinal nerves. 



The medulla oUojigata is the prolongation of the spinal cord, extend- 

 ing to the pons Varolii. This portion of the brain is very large in the 

 horse; it is pyramidal in shape, the narrowest part joining the cord. 



ThQ2>ons 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 greatest. The body of the 

 cerebellum is composed of gray matter externally and of white matter 

 in the center. The cerebellum has the function of coordinating move- 

 ments, that is, of so associating them as to cause them to accomplish 

 a definite purpose. Injuries to the cere})ellum cause disturbances of 

 the equilibrium, but do not interfere with the will power or intelligence. 



