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TEXT-BOOK OF PHYSIOLOGY. 



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 multi- 

 polar. They are the modified descendants of pear-shaped cells, the neuro- 

 blasts, which migrated from the medullary tube (see page 95). In the course 

 of their migration they developed dendrites which form an intricate felt- 

 work throughout the anterior horn. One of the processes, the axon, ap- 

 proached the surface of the cord, penetrated it, 

 grew outward, became covered with myelin and 

 neurilemma, and developed into an anterior 

 root-fiber. These nerve-cells, with their den- 

 drites, axons, and terminal branches, form 

 efferent neurons of the first order. The inti- 

 mate histologic and physiologic relationship 

 existing between the nerve-cell and the axon is 

 revealed by the degenerative changes which 

 arise in the latter when separated from the 

 former. The cell apparently determines the 

 B nutrition of the axon and may be regarded as 

 trophic in function. Some of the cells of the 

 anterior horn send their axons into the im- 

 mediately surrounding white matter of the same 

 side, after which they divide into two branches, 

 one passing up, the other down, the cord, to 

 re-enter the gray matter at different levels. 

 They are probably associative in function. 

 C Other cells send their, axons into that portion 

 of the white matter on the same and opposite 

 sides known as Gower's antero-lateral tract. 

 (Fig. 231.) 



In the posterior horn nerve-cells are also 

 present, though they are not so numerous as 

 in the anterior horn. At the base of the horn 

 and on its inner side there is a well-marked 

 D group of cells which extends from the seventh 

 or eighth cervical nerves downward to the sec- 

 L or third lumbar nerves, being most promi- 

 OF THE nent in the thoracic region. This column is 

 SPINAL CORD. A. At the level known as Clarke's vesicular column. From the 



of the sixth cervical nerve. B. 



At the mid-dorsal region, c. nerve-cells constituting this column axons pass 



At the center of the lumbar obliquely outward into the portion of the white 



part'oMhe* comis ^meduiiaris. PP i! matter known as the direct cerebellar tract. 



Posterior roots. 2. Anterior roots! Other nerve-cells send their axons into the white 



3. Posterior fissure. 4 . Anterior ma tter in the posterior portion of the cord 



fissure, c. Central canal. (Mor- , , . .-, T r o 



ris' "Aruitomy," after Schwaibe.) bordering the posterior median fissure. Some 



of the nerve-cells, their situation and the dis- 

 tribution of their axons are shown in Fig. 231. 



Classification of Nerve-cells. The cells of the gray matter may be 

 divided into three main groups: viz., intrinsic, efferent, and afferent. 



FIG. 230 

 DIFFERENT 



SECTIONS THROUGH 

 REGIONS 



