THE NATUEE OF THE BKAIN. 



513 



part represents the rudiment of the encephalon or brain ; and the rest of the tube 

 will become converted into the medulla spinalis. 



If the attempt is made to analyse the meaning of the early broadening of the 

 brain rudiment it will be' found to be due in great measure to the fact that there 

 is added to the margins of the medullary plate (see Fig. 442, E, p. 501) the material 

 from which the sensitive part of the eye and the optic nerve will be developed ; but 

 soon after the neural tube is closed irregular swellings will make their appearance 

 around the attachments of the nerves of smell, vision, hearing, and taste (Fig. 454), 



Optic tract 



Tectum mesencephali 



I 



I Red nucleus 



Tecto- spinal tract 



Ik. ^ Rubro-spinal tract 



^^.Brachium conjunctivum 



.-- Leraniscus medialis 



'Lemniscus lateralis 

 Cerebellum 



Vestibulo-spinal tract 

 Nucleus gracilis 



i- spinal ^ 



tract - 

 Olfactory nerve 



icates the place where a tract crosses the median plane. 



Medulla oblongata 



Fibres of 



- posterior 



funiculus 



)4. DIAGRAM REPRESENTING THE CONNEXIONS OP .SOME IMPORTANT SKNSORY AND MOTOR TRACTS 

 IN THE BRAIN to which references are made in pages 513 to 517. Motor paths in red ; sensory in other 

 colours. 



( and also the great vagus nerve that is widely distributed to the viscera of the neck, 

 thorax, and abdomen. 



But there are other factors besides these irregularities of growth of its walls 



which add complexity to the form of the encephalon in the embryo. In the 



course of their growth both parts (encephalon and medulla spinalis) of the neural 



ube undergo great extensions in length, breadth, and thickness ; but in the case 



the spinal medulla it is the increase in length that is most distinctive, whereas 



,in the encephalon, the irregular expansion in breadth and thickness is more 



obtrusive. Nevertheless, the brain elongates more rapidly than that part of its 



,mesodermal capsule which ultimately becomes the brain-case or cranium; and 



lence it becomes bent to permit of its being packed in the limited length of the 



Cranial cavity. But if it is admitted that these mechanical considerations are 



in a measure responsible for the three bends which develop in the embryonic 



^ncephalon, their situation and the forms they assume are determined by the 



^regularities of growth inherent in the brain itself. 



34 



