THE SPINAL CORD 533 



their development push the inner wall of the dura ahead of them and 

 thus come to project directly into the blood stream of the superior longi- 

 tudinal and other sinuses. The thin walls of the villi permit of the passage 

 of the fluid into the blood stream. 



The function of this fluid has been variously interpreted. Thus it 

 has been supposed by Halliburton to be a nutritive fluid playing the role 

 in the nutrition of the brain, that lymph does in the nutrition of other 

 organs and tissues of the body. This view is based on the assumed fact 

 that as there are no lymph vessels detectable in the brain tissue, there is 

 therefore no lymph produced in the brain as in other regions of the body. 

 While it plays the part of lymph, it differs from lymph found elsewhere in 

 the body in its mode of origin and chemical composition. In addition 

 to the food constituents it contains oxygen and at least one katabolic 

 product, carbon dioxid. On the other hand it is assumed by others that 

 the function is partly physical; being placed between the walls of the 

 skull and the spinal column on the one hand, and the brain and spinal 

 cord on the other hand, it acts as a water cushion protecting these delicate 

 organs from the injury that might result from sudden jars. 



The Functions of the Nerve System. It will be recalled that the 

 entire nerve system can be resolved into simple morphologic units, termed 

 neurones, each of which possesses certain histologic features and physiologic 

 properties ; that their relation one to the other both in a serial and collateral 

 manner gives rise to the general architecture of the nerve system. Recalling 

 the functions of neurons in their individual and collective capacities the 

 functions of the nerve system may be formulated. 



The functions of the nerve system are twofold: (i) It unites and asso- 

 ciates the organs and tissues of the body in such a manner that they are 

 enabled to cooperate for the accomplishment of a definite object. (2) It 

 serves to arouse in the individual a consciousness of the existence of an 

 external world, by virtue of the impressions which it makes on his sense 

 organs, and consequently to enable him to adjust himself to his environment. 



By virtue of the anatomic and physiologic association, a stimulus if of 

 sufficient intensity, applied to one organ or tissue will call forth activity 

 in 'one or more organs near or remote from the part stimulated. This 

 coordination of action is accomplished mainly by the spinal cord and the 

 medulla oblongata. All actions which take place in response to a periph- 

 eral stimulus and independently of volition are termed reflex actions. The 

 reflex activities connected with digestion, the circulation of the blood, with 

 respiration, excretion, etc., are illustrations of the coordinating capabilities 

 of the nerve-centers located in these portions of the central nerve system. 



By virtue of the physiologic activities of the encephalon and more 

 particularly of the cerebrum and of the relation existing between it and the 

 sense organs of the body, consciousness of the existence of an external 

 world and the individual's relation to it is developed. Experimental and 

 clinic investigations show that of the parts of the encephalon, the cerebrum 

 is the chief, though not perhaps the sole organ of the mind and that its 

 functions are for the most part mental. 



The function of a part at least of the peripheral nerve system is to afford 

 a means of communication between the central organs of the nerve system 

 and the remaining structures of the body. 



