260 INNERVATION. [CHAP. X. 



in the white matter, and the observer should be careful not to con- 

 found the most minute of them with some of the fibrous elements 

 above described. 



So far as our present knowledge of the minute anatomy of the 

 spinal cord extends, it is favourable to the supposition that the spinal 

 nerves derive their origin, at least partly, from the gray matter. 

 The longitudinal fibres of the cord may consist in part of fibres 

 continuous with those of the brain or cerebellum, and in part of 

 commissural fibres, serving to unite various segments of the cord 

 with each other, or to connect some part or parts of the encephalon 

 with them. Those fibres which may be regarded as strictly spinal 

 are probably oblique in their course, forming their connexion with 

 gray matter at a point higher up in the cord than that at which 

 they emerge from its surface, and may be readily confounded with 

 the longitudinal fibres when their course is long. Other oblique 

 or transverse fibres probably do not emerge from the cord, but 

 connect the segments of opposite sides, forming a transverse com- 

 missure. So that four classes of fibres, each different in function, 

 may be considered to exist in the cord. 1. Spinal fibres, oblique 

 or transverse, which propagate nervous power to or from the seg- 

 ments of the cord itself. 2. Encephalic fibres, longitudinal, the 

 paths of volition and sensation, which connect the spinal cord with 

 the various segments of the encephalon. 3. Longitudinal commis- 

 sural fibres. 4. Transverse commissural fibres. 



Of the Encephalon. The brain or encephalon is the mass which 

 is contained within the cranial cavity. The plane of the occipital 

 foramen separates it from the spinal cord, inasmuch as that plane 

 would about pass through the inferior extremity of the medulla 

 oblongata. 



Four segments are obviously distinguished in the encephalon. 

 1. The medulla oblongata. 2. The cerebrum. 3. The cerebellum. 

 Some fibres of the medulla oblongata extend to the cerebrum, and 

 some to the cerebellum. The fourth segment, which is called the 

 mesocephale, contains fibres passing between all the rest, as well 

 as some connecting opposite sides. This constitutes a sort of 

 conflux to the segments above named, and may be compared to 

 a railway terminus, at which several lines meet and pass each 

 other. 



The brain of the adult man weighs about 50 oz. or a little 

 more than 3 Ibs. avoirdupois.* This great weight depends mainly 



* See Reid's Tables. Lond. and Edinb. Monthly Journal of Med. Science. 

 Ap. 1843. 



