THE ENCEPHALON OK BKAIN. . 539 



The fasciculus cerebrospinalis anterior (O.T. direct pyramidal tract) is usually a 



j rve-strand of small size which lies near the anterior median fissure. As a rule it 



( >inot be traced lower than the middle of the thoracic region of the spinal medulla. 



is a descending tract and must be associated with the lateral cerebro-spinal 



: sciculus of the opposite side, seeing that both of these strands arise from the motor 



j3a of the cortex of the same cerebral hemisphere. From this it must be clear 



at the anterior cerebro-spinal fasciculus does not cross the median plane as it 



- ters the spinal medulla, but descends on the side of the spinal medulla corre- 



onding to the cerebral hemisphere in which it arises. Nevertheless, its fibres do 



>t end in the same side of the spinal medulla, but at every step along the path of 



e strand they make use of the anterior commissure, and cross to the opposite side 



the spinal medulla, to terminate in relation to the opposite ventral motor cells 



the same manner as the lateral cerebro-spinal fibres. 



From this crossing of the cerebro-spinal fasciculi, it follows that the destruction of the fibres 

 :iich compose them as they descend in one side of the brain must result in paralysis of the 

 uscles supplied by the efferent nerves of the opposite side of the spinal medulla. 



In cases of old brain lesion it is sometimes possible to detect some degenerated fibres in the 

 teral cerebro-spinal fasciculus of the sound side of the spinal medulla, and from this it is 

 pposed that this tract contains a few uncrossed fibres. If this is the case, each side of the 

 inal medulla stands in connexion with the motor area of both cerebral hemispheres. 



It is well to note that the fibres of both lateral cerebro-spinal fasciculi are not medullated 

 itil the time of birth. They are the latest of all the fasciculi of the spinal medulla to myelinate. 



Commissura Anterior Alba. The anterior white commissure is composed of 

 ledullated nerve-fibres passing from one side of the spinal medulla to the other 

 nd entering the anterior column of gray matter, and also the anterior funiculus of 

 r hite matter. It is to be regarded more as a decussation than as a commissure, 

 nd its width, which varies somewhat in different regions, fluctuates in correspond- 

 Qce with the diameter of the spinal medulla. 



Amongst the fibres which .cross in the anterior commissure may be mentioned : (1) The fibres 

 f the fasciculus cerebrospinalis anterior ; (2) collaterals from both the anterior and lateral 

 miculi ; (3) axons of many of the cells of the gray matter ; (4) the dendritic processes of some 

 f the medial anterior cells. 



Commissura Grisea. Although this is composed of gray matter with a large 

 Admixture of neuroglia, numerous nerve-fibres pass transversely through it, so as 

 o establish relations between the cells in the gray matter on the two sides of the 

 pinal medulla. 



THE ENCEPHALON OR BRAIN. 



The brain is the enlarged and greatly modified upper part of the cerebro-spinal 

 nervous axis. It is surrounded by the same membranes that envelop the medulla 

 spinalis (viz., the dura mater, the arachnoid, and the pia mater), and it almost 

 Completely fills up the cavity of the cranium. So closely, indeed, is the skull 

 ! capsule moulded upon the brain that the impress of the latter is almost everywhere 

 evident upon the inner surface of the cranial wall. The relations, therefore, of 

 cranium to brain are totally different from those presented by the vertebral canal 

 the spinal medulla. As we have noted, the medulla spinalis occupies only a 

 part of its bony case ; and there is not only a wide and roomy space between the 

 ichnoid and the pia mater, but also an interval of some width between the 

 dura mater and the walls of the vertebral canal. 



General Appearance of the Brain. When viewed from above the brain 



'Sents an ovoid figure, the broad end of which is directed backwards. Its 



t transverse diameter is usually found in the neighbourhood of that part 



lies between the two parietal tuberosities of the cranium. The only parts 



i are visible when the brain is inspected from this point of view are the two 



avoluted cerebral hemispheres. These present an extensive convex surface, which 



