188 



The nerve fibers represent a conducting apparatus, and serve to 

 place the central nervous organs in connection with peripheral end 

 organs. The nerve cells, however, l)esides 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 Inindle of tuljular fillers, held together by a dense areolar 

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

 fibers possess no elasticit}^, 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 irritahJe propertiefi. 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 imi^ressionsto the centers are termed sonsory 

 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 simj)lest terms, as follows: It is intended to associate the different 

 parts of the hody insiadi a manner that sfinndus 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 j)07?.s Varolii, and the cerebrum, and it is cov- 

 ered by three membranes called the meninges. The first of these 

 membranes, the dura mater, is a tMc'k, white, fibrous membrane which 

 lines tlie cavity of the cranium, forming the internal i3eriosteum 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 

 loosel}^ envelops the l)rain and spinal cord; it forms two layers, leav- 

 ing between them the arachnoid space which contains tlife cerebro- 

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

 from pressure. The third, the jji'a mcder, 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, ex- 

 tending 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 pons Varolii is the transverse projection on the base of the 

 brain, between the medulla oblongata and the peduncles of the cere- 

 brum. 



The cerebellum is lodged in the posterior part of the cranial cavity, 

 immediately above the medulla oblongata; it is globular or elliptical 



