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organs. The nerve cells, however, besides transmitting impulses, act 

 as physiological centers for automatic or reflex movements, and also 

 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. Iv'erve 

 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 i^ossess excitable or irritable ])roperties. 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 x>arts 

 of the body in siich a manner that stimulus applied to one organ may ex- 

 cite 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, t\iQ pons Varolii, and the cerebrum, and it is covered by three 

 membranes called the meninges. The first 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 con- 

 tinuous with the spinal cord to the extremity of the canal. The second, 

 the araclinoid, 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, 

 the pia mater, is closelj' 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, 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. 



The ^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 cere- 

 bellum is composed of gray matter externally and white in the center. 



The cerebrum, or brain proper, occupies the anterior portion of the 



