THE SPINAL CORD. 



445 



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The anterior horn.. is. short and broad and entirely surrounded 

 by white matter. The posterior horn is narrow and elongated and 



extends quite up to the surface of the 

 cord, where it is capped by gelatinous 

 matter, the subsiantia gelatinosa. In 

 the lower cervical and thoracic regions 

 a portion of the intermediate gray 

 substance projects outward and forms 

 the so-called lateral horn. The gray 

 nrafjgr fundamentally coijgjgJ^H a 

 framework of fine neuroglia supporting 

 blood-vessels, lymphatics, medullated 

 and non-medullated nerves, and groups 

 of nerve-cells. 



The Nerve-cells. The nerve-cells 

 of the cord are very numerous and 

 present a variety of shapes and sizes' in 

 different regions. They are usually ar- 

 ranged in groups which extend for some 

 distance upland down the cord, forming 

 columns more or less continuous. 



In the antenor. horn two well- 

 marked groups are found, one situated 

 at the anterior and inner angle, known 

 as the antero-median group, the other 

 situated at the posterior and lateral 

 angle and known as the postero-lateral 

 group. In the lower cervical and upper 

 thoracic regions, in the region of the 

 lateral horn, another group of cells 

 is found, known as the intermediate 

 group. In the central portion of the 

 horn there is also a central group. 



The cells of the anterior horns are 

 of large size, nucleated and multipolar. 

 They are the modified descendants 

 of pear-shaped cells, the neuroblasts, 

 which migrated from the medullary 

 tube (see page in). In the course of 

 their migration they developed den- 

 drites which form an intricate felt- 

 work throughout the anterior horn. 

 One of the processes, the axon, 

 approached the surface of the cord, penetrated it, grew outward, 

 became covered with myelin and neurilemma, and developed into 



D 



FIG. 204. SECTIONS THROUGH 

 DIFFERENT REGIONS OF THE 

 SPINAL CORD. A. At the 

 level of the sixth cervical 

 nerve. B. At the mid-dorsal 

 region. C. At the center of 

 the lumbar enlargement. D. 

 At the upper part of the 

 comis medullaris. i. Poste- 

 rior roots. 2. Anterior roots. 

 3. Posterior fissure. 4. Ante- 

 rior fissure. 5. Central canal. 

 (Morris' "Anatomy," after 

 Sch-walbe.) 



