280 NERVE-TISSUE. 



It is only the accompanying and ensheathing connective tissue 

 which carries vessels. 



In order to facilitate the study of the nervous system, it is 

 convenient to subdivide it into three main portions, all of which 

 form one continuous mass of tissue i. e., the central portion, 

 the conducting portion, and the terminal portion. 



1. NERVE-CENTERS. 



In the brain and the spinal cord, it is only the gray sub- 

 stance which can be called nerve-center, as the white substance is 

 composed of conducting nerves. The ganglia of the sympathetic 

 nerves may also be considered as central organs. 



(A)' Brain. According to Th. Meynert,* the gray substance of the brain 

 may be divided into four groups : 



(a) The uppermost mass of gray matter, from which the entire white sub- 

 stance of the brain takes its rise, is the superficial gray substance of the cerebral 

 lobes the cortex cerebri; 



(b) Collections of gray matter are the ganglia of the cerebrum the corpus 

 striatum, the three members of the lenticular nucleus, the thalamus opticus, 

 and the corpora quadrigemina ; 



( c) The tubular gray matter, which invests the cavities of the brain as a 

 direct prolongation of the gray substance of the spinal cord, traceable through 

 the fossa rhomboidalis, the aqueduct, the middle ventricle, into the tuber 

 cinereum and the infundibulum ; 



(d) The gray substance of the cerebellum, arranged partly in layers, partly 

 in central aggregations, and in connection with the gray formations of the 

 caudex cerebri, which are traversed by the medullary substance of the cere- 

 bellum. 



The gray masses are connected by means of fibrous tracts of the white 

 substance. The gray cortex of the brain receives all the sensory impressions 

 from the outer world, and from it arise the motor impulses, communicated to 

 the motor nerves. 



The medullary or white substance of the large hemispheres of the brain 

 the corona radiata furnishes the routes of this system of projections of the 

 first order ; and, moreover, this system is connected by bundles of nerve-fibers 

 with the cerebellum. The fibers of this projection system take mostly a radi- 

 ating course ; additional formations of this system are the commissures in the 

 corpus callosum, which connect the corresponding portions of the right and 

 the left side, and the systems of association, which connect different regions 

 of the cortex in one hemisphere. 



The ganglia of the cerebrum interrupt the projection system of the first 

 order, and in them a reduction of the number of the fibers of this system 



* " A Manual of Histology," by S Strieker. American edition, 1872. 



