116 DISEASES OF CATTLE. 



The foregoing revifew of the cranial nerves and their functions, brief 

 as it is, will give a superficial idea of the uses of the nervous system 

 and the magnitude of its importance. 



The meninges are the membranes, three in number, which envelop 

 the brain and spinal cord, and separate them from the bones which 

 form the walls of the cranial cavity and spinal canal. These mem- 

 branes are called the dura mater, external; the arachnoid, middle; 

 and the pia mater, internal. 



The dura mater is composed of very strong and dense tissue. By its 

 external surface it adheres more or less closely to the bones which form 

 the walls of the cranial cavity and is continued throughout the whole 

 length of the spinal canal, but does not adhere to the bones of the 

 spinal canal to the same extent as in the cranial cavity. 



The arachnoid is a serous membrane, and, like all serous membranes, 

 has two layers, which form a closed sack. The external layer is in con- 

 tact with the dura mater, and the internal layer is in contact with the 

 pia mater. The inner surfaces of the arachnoid (the interior of the 

 sack) are in contact, and are kept moist by the exudation of serum. In 

 the cranial cavity the external layer is not closely attached to the dura 

 mater, but in the spinal canal they are closely united. 



The pia mater, which is in direct contact with the brain and spinal 

 cord, is a very thin, delicate membrane, having in its structure many 

 blood vessels and small nerves. This membrane, being intimately at- 

 tached to the nervous matter of the brain and spinal cord, follows all 

 the convolutions, dipping down into the various fissures and inequalities. 

 The arachnoid does not dip into the inequalities, and consequently 

 spaces are left between it and the pia mater. These spaces are filled 

 with a fluid called the subarachnoid fluid, in which the brain and spinal 

 cord may be said to be immersed. This fluid is of vast importance, 

 in so far as it, to a great extent, prevents concussion to the nervous 

 matter of the brain and cord. 



The sympathetic, also called the ganglionic, division of the nervous 

 system consists of two chains of ganglia, reaching from the head to 

 the tail, situated beneath the spinal column, one on either side. The 

 presence of the ganglia or enlargements on the cords give them their 

 chain-like appearance. 



As previously remarked, the sympathetic nerves are closely connected 

 with the cerebro-spinal nerves. In fact, the center for the sympathetic 

 system is located in that part of the brain called the medulla oblongata, 

 but the sympathetic nerves are not under the control of the will. 

 Afferent nerves come from the cerebro-spinal system, as pointed out 

 when describing the spinal nerves. Efferent nerves go from the ganglia 

 (or nerve centers) to all the blood vessels, various organs in the neck, 

 chest, abdomen, etc. The ganglia belonging to the sympathetic system 

 are numerous, and it is thought by some physiologists that they par- 

 ticipate in both automatic and reflex acts. 



