194 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



Chick embryos, hatched under abnormal conditions as regards 

 temperature, frequently show a cystic condition of the neural 

 tube, which is also accompanied by a dropsical state of the 

 mesoblastic tissues. If the cystic condition occurs at an early 

 stage it may dilate the fore-brain, or even the whole cephalic part 

 of the tube until it bursts. This is probably the pathology of 

 the condition seen in anencephalic foetuses, children born with 

 all the parts of the body developed except the brain and roof 

 of the skull, which are represented merely by a broken mass 

 of tissue. A cystic condition of the lateral ventricles, which 

 are formed as diverticula of the fore-brain, occasionally occurs 

 towards the end of foetal life, and gives rise to the condition 

 known as hydrocephalus. 



Encephalocele and Spina-bifida. — Localised dilatations of the 

 neural tube may occur. The most common, spina bifida, occurs 

 in the lumbar region, where the medullary plates are last to 

 close. The dilatation may affect the arachnoid and dura mater 

 only ; or the neural tube may be also distended. Another site 

 is at the anterior or cephalic end of the tube, where the 

 medullary plates are also late in closing. The encephalocele, 

 or it may be only a meningocele, formed in this site, projects at 

 the root of the nose or within the nasal cavity. A meningocele 

 may also occur at the roof of the 4th ventricle ; it projects at the 

 occipital fontanelle (Fig. 132, p. 166). 



The Spinal Cord. — The Spinal Cord at first extends the whole 

 length of the spinal column. After the 4th month the spinal 

 column and canal grow more rapidly than the cord, and at birth 

 its lower end has become withdrawn to the level of the 3rd 

 lumbar vertebra. By the third year it only reaches the disc 

 between the 1st and 2nd lumbar vertebrae. The results of this 

 inequality of growth, are — 



(1) The roots of the lumbar and sacral nerves become enor- 

 mously elongated, forming the cauda equina ; all the nerves are 

 more or less drawn up, except the 1st and 2nd cervical ; the 

 origins of the lower cervical nerves are drawn up 2 vertebrae (as 

 indicated by the position of their spines) ; the upper dorsal, 3 ; the 

 lower dorsal, 4 ; the lower lumbar, 5 ; the coccygeal, 10. These 

 statistics represent a broad expression of the observations made 

 by Professor E. W. Eeid. j 



