THE NERVOUS SYSTEM. 



783 



These may be divided into : 



1. Amyelia, or absence of the spinal cord. This is almost invariably associated 

 with absence of the brain. 



2. Atelomyelia, or partial development of the spinal cord. This is often seen in the 

 anencephalous or acephalic monsters, where, corresponding to the incompletely developed 

 brain, there may be various degrees of defective development in the length of the cord. 



3. Diastematomyelut, a condition in which a portion or the whole of the cord is 

 split into two lateral halves. Each half of the cord, being enveloped in its own mem- 

 branes and giving rise to its own nerve roots, may fuse together to form a single cord 

 at some region. 



4. Diplomyelia, or a formation of two spinal cords a duplication of the spinal cord. 

 This happens in the various kinds of double monsters. 



II. MINOR CONGENITAL MALFORMATIONS NOT INCONSISTENT WITH THE MAINTE- 

 NANCE OF LIFE. 



1. Hydrorrhacfiis internet, is a defective closure or arrangement of the divisions of the 

 primary fcetal central canal often resulting in the dilatation of the central canal by fluid 

 (Hydromyelia) (Fig. 514). This dilatation may be moderate, or so extreme that but 

 little of the substance of the cord is left as a thin shell around the central cavity. When 

 they have not been destroyed by atrophy, epithelial cells may be found lining the cavity. 



FIG. 514. HYDUOMYELTA. . 



In the section from which this drawing was made, the epithelial cells surrounding the dilated central canal 

 were well preserved. 



This condition may be accidentally found after death. Its presence may also be 

 indicated by its association with spina bifida. 1 



2. Heterotopia, or misplacement of the substances of the cord. 

 (a) There may be misplaced portions of the gray matter. 



(6) Portions of the white matter may be arranged in an unusual manner. 



3. Anomalies of the Spinal Nerve Roots. 



4. Asymmetries of the Spinal Cord. 



III. MALFORMATIONS OF THE SPINAL CORD ACQUIRED DURING EXTRA-UTERINE 

 LIFE OR SECONDARY TO DEFECTIVE DEVELOPMENT IN OTHER PARTS on THE BODY. 



1. Distortions following other cord lesions. 



2. Asymmetry of the cord due to arrested development after birth or to secondary 

 atrophy of portions of the cord in association with defective development or absence of 

 some other part of the body. 



1 Under this subdivision the condition known as hydrorrhachis extcrna may be con- 

 veniently alluded to, which consists in an abnormal congenital accumulation of fluid 

 between the meninges of the cord, causing more or less diminution in the volume of the 

 latter. 



