THE MEDULLA OBLONGATA AND CEREBELLUM 145 



A functional system may be defined as the sum of all the 

 neurons in the body which possess certain physiological and 

 anatomical characters in common so that they may react in a 

 common mode. Morphologically, each system of peripheral 

 nerves is defined by the terminal relations of its fibers by the 

 organs with which they are related peripherally and by the 

 centers in which the fibers arise or terminate. A single periph- 

 eral nerve may contain several of these systems. It becomes 

 necessary, therefore, to analyze the root complex of each pair of 

 spinal and cranial nerves into its components, and to trace not 

 only the central connections of these components within the 

 spinal cord and brain, but also their peripheral courses as well. 

 In other words, the description of any given nerve or ramus is not 

 complete when we have given its point of origin from the nerve- 

 trunk, root, or ganglion, the details of its devious courses, and 

 the exact points where the several ramuli terminate. In addi- 

 tion to this it is necessary to learn what functional systems are 

 represented in each ramus and the precise central and peripheral 

 relations of each system. 



Each of the four primary divisions of the spinal nerves 

 (somatic sensory and motor, visceral sensory and motor, see 

 p. 126) is represented in the head region in the same primitive 

 unspecialized form as seen in the spinals, and also by specialized 

 systems found only in one or more cranial nerves. This gives 

 eight groups of functional systems represented in the cranial 

 nerves, as follows: 



1. General somatic afferent nerves, supplying (1) general exteroceptive 

 sensibility to the skin and the underlying tissues, and (2) deep propriocep- 

 tive sensibility to the muscles, tendons, etc. Type 1 is represented in the 

 V, IX, and X nerves, and in some lower vertebrates in the VII nerve also 

 (there is some clinical evidence for its presence in the VII nerve of man) ; 

 type 2 is represented in the III, IV, V, VI nerves and probably in some of 

 the others also. 



2. Special somatic afferent nerves, for the innervation of highly differ- 

 entiated sense organs. Here belong in the exteroceptive series the coch- 

 lear branch, and in the proprioceptive series the vestibular branch of the 

 VIII pair. The lateral line nerves of fishes belong here, and probably the 

 visual organ connected with the II pair in all vertebrates (though the so- 

 called optic nerve is not a true nerve, see p. 204). 



3. General somatic efferent nerves, supplying the general skeletal muscu- 

 lature of the body. In fishes this system is represented in several cranial 

 nerves in addition to the spinalis, but in man it is lost in the cranial nerves, 

 unless, as some believe, a part of the fibers of the XI pair belong here. 



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