DISEASES OF THE NERVOUS SYSTEM. 191 



for the sensory, perceptive, trophic, and secretory functions. A 

 nerve consists of a bundle of tubular fibers, held together by a dense 

 areolar tissue, and inclosed in a membranous sheath — the neurilemma. 

 Nerve fibers possess no elasticity, but are very strong. Divided nerves 

 do not retract. 



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 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 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 cra- 

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

 the cerebellum, the pons Varolii, and the cerebrum — and it is cov- 

 ered by three membranes, called the meijinges. The outer of these 

 membranes, 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 arachnoid, 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- 

 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 adher- 

 ent 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, 

 hnmediiately above the medulla oblongata; it is globular or elliptical 



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