BRAIN 199 



dorsal (sensory) and ventral (motor) nerve-roots to be respectively 

 connected together by a longitudinal line, each half of the spinal 

 cord would thus be divided into three columns, a ventral, lateral, 

 and dorsal. Anteriorly the cord becomes continuous with the 

 posterior portion of the brain (medulla oblongata), with the 

 ventricle of which the central canal communicates. 



As regards its minute structure, two kinds of nervous tissue can 

 be distinguished in the spinal cord, a white substance, consisting 

 of nerve-fibres only, and a gray substance, composed of nerve-cells 

 as well as fibres. 1 Their relative positions vary in the various 

 vertebrate groups, as well as in different regions of the cord ; the 

 white substance, however, has in general a more peripheral, the gray 

 a more central position, the latter usually presenting a pair of 

 dorsal and ventral cornua in transverse section, its groups of cells 

 showing in many places a metameric arrangement. 2 



2. The Brain. 



Before the medullary groove becomes closed, three swellings 

 may be seen at the anterior enlarged part of the medullary tube : 

 these are spoken of as the primary anterior, middle, and posterior 

 cerebral vesicles, or fore-, mid-, and hind-brain. (Fig. 148). The 



FIG. 148. DIAGRAM OF THE EMBRYONIC CONDITION OF THE CENTRAL NERVOUS 



SYSTEM. 



G, brain, with its three primary vesicles. 7, //, /// ; R, spinal cord. 



cavities of the vesicles are in direct connection with the central 

 canal of the spinal cord. 



A differentiation of the primary fore-brain and hind-brain 

 respectively into two parts then takes place, and th us five divisions 

 of the brain may be distinguished. Counted from before back- 

 wards these are the telenccphalon (secondary fore-brain or cerebral 

 rudiment), diencephalon (primary fore-brain or twixt-brain), mes- 

 encephalon (mid-brain), metencephalon (secondary hind-brain), and 



| Peculiar structures, known as Reissner's fibres, arising in the brain and ex- 

 tending along the spinal cord, occur in all vertebrate Classes. They represent an 

 ancient but highly differentiated apparatus, the function of which is to pass optic 

 stimuli by reflex action to the motor tracks of the cord. These fibres are largest 

 in Elasmobranchs and Teleosts, and are reduced or wanting in blind animals and 

 those in which the eyes have undergone reduction. 



- In Struthioua Birds a row of elevations occurs in the liunbo-sacral region, 

 recalling the metameric segmentation of the cord in the Teleost Tr'ujln. The origin- 

 ally metameric character of the cord is also indicated phylogenetically and onto- 

 genetically by the primary, segmental blood-vessels. Tin- dorsal and ventral 

 longitudinal vascular trunks arise secondarily. 



