SPINAL CORD 



637 



bony arches that they may escape injury, even 

 when the bodies of several dorsal verteurte have 

 been destroyed by alceration. 



The spinal cord is that part of the central 

 nervous axis which is contained within the verte- 

 bral or spinal canal. In man it extends from the 

 foramen magnum, where it becomes continuous 

 with the medulla oblongata, as far down as the 

 lower border of the first lumbar vertebra. Below 

 this point it is continued as a narrow thread con- 

 taining little nervous matter, the filum terminate. 

 The cord is, like the brain, encased in a triple 

 sheath of membranes, the pia, arachnoid, and dura 

 mater, the last two of which are less closely 

 opposed to the cord and the vertebral column 

 than they are to the brain and the cranium. The 

 cord is a somewhat flattened cylinder or column of 

 a whitish colour. It is divided into two lateral 

 halves by an anterior median fissure and a pos- 

 terior median septum. The former is less deep but 

 wider than the latter, which is merely a process 

 of pia mater. From each side of the cord thirty- 

 one pairs of spinal nerves arise, each by an anterior 

 and a posterior root (see NERVOUS SYSTEM). The 

 cord is not of uniform circumference throughout. 

 There is a slight thickening where the brachial 

 nerves, and another where the lumbar nerves, 

 arise. These are termed the brachial or cervical 

 and lumbar enlargements respectively. The nerve 

 roots in the cervical and dorsal regions pass nearly 

 transversely outwards through 

 the intervertebral foramina ; but 

 the lumbar and sacral nerves 

 are directed downwards to 

 attain their points of exit, in 

 such a manner as to obtain the 

 name of Cttutla Equina. 



The cord is seen to be com- 

 posed of white and f^ray matter. 

 On a transverse section the gray 

 matter is found in the centre 

 to present the appearance of a 

 double crescent, concave out- 

 wards, with a cominissural band 

 l>etween the two sides, so that 

 the whole somewhat resembles 

 the letter H. In the gray com- 

 missure is the central canal, 

 which extends throughout the 

 whole length of the cord and 

 becomes continuous above with 

 the fourth ventricle. The gray 

 matter is divided into an an- 

 terior and a posterior horn. It 

 is composed of a basis of con- 

 nective tissue in which are a 

 fine felt work of nerve-fibres 

 and nerve-cells. In the anterior horn, which be- 

 comes specially developed in the brachial and 

 Inmliar enlargements, the cells are of the multi- 

 polar type. They send a well-marked axis cylinder 

 process into the anterior nerve root, and by their 

 other processes are connected with fibres which 

 descend from the cerebrum and from the posterior 

 roots and other sources. In the posterior horn the 

 nerve-cells are smaller, rounded, or spindle-shaped, 

 and possess no axis cylinder process. They are 

 probably connected with the posterior roots. 



The white matter of the cord is composed of 

 nerve-fibres, united by a small amount of con- 

 nective tissue. The fibres have mostly a longi- 

 tudinal direction, with the exception of a small 

 strand which crosses from one side to the other at 

 the bottom of the anterior median fissure the an- 

 terior white commissure. Each half of the cord 

 in divided by the anterior and posterior roots into 

 three columns, named anterior, lateral, and pos- 

 terior. Though these look almost exactly alike 



both to the naked eye and under the microscope, 

 they can, by taking advantage of the facts that 

 their component fibres are not all developed simul- 

 taneously, and that as the result of injury they 

 do not ' degenerate ' in the same direction or to the 

 same extent, be further subdivided. Thus the 

 posterior column is composed of two tracts a 

 postero-median and a postero-lateral. In the 

 lateral column there can be distinguished five 

 tracts : a direct cerebellar, an ascending antero- 

 lateral tract, a crossed pyramidal tract, a lateral 

 limiting layer, and a lateral mixed zone ; while the 

 anterior column consists of a direct pyramidal 

 tract and an anterior mixed zone. The relations 

 of these tracts have been fairly definitely estab- 

 lished. The postero-median column is nearly 

 entirely composed of fibres of the posterior roots 

 which, after entering the cord and passing for a 

 short distance in the postero-external column, are 

 continued upwards as high as the medulla oblon- 

 gata, where they end. This column then will form 

 a direct path of connection of the posterior roots 

 with the medulla oblongata. The direct cerebellar 

 tract connects a column of nerve-cells in the pos- 

 terior horn of the lumbar and dorsal regions of the 

 cord (which is termed Clarke's vesicular column) 

 with the middle lobe of the cerebellum. The 

 antero-lateral tract appears to connect the nerve- 

 cells in the posterior horn of one side of the cord 

 with the medulla oblongata of the opposite side. 



Fig. 2. Section of Spinal Cord (outline of tracts shown on left side) : 

 1, anterior median fissure ; 2, posterior median septum ; 3, 4, 5. anterior, lateral, and 

 posterior white columns ; 6, 7, anterior and j>osterior horn nf gray matter ; 8, 9, anterior 

 and posterior commissures ; 10, posterior median column; 11, posterior lateral column ; 

 12, direct cerebellar tract ; 13, antero-lateral ascending tract ; 14, crossed pyramidal 

 tract ; 15, lateral limiting layer ; 16, 17, lateral and anterior mixed zone ; 18, anterior 

 pyramidal tract ; 19, posterior, and 20, anterior roots. 



All these tracts degenerate, and presumably also 

 conduct nervous impulses in an upward direction. 

 The direct and crossed pyramidal tracts form the 

 downward continuation of the 'motor '-fibres from 

 the surface of the brain to the cells in the anterior 

 cornu of the cord. At the lower end of the 

 medulla oblongata (see BRAIN, fig. 7) the greater 

 part of the fibres of the anterior pyramids cross 

 the middle line and descend in the crossed pyra- 

 midal tract of the cord to become connected with 

 the motor-cells in the anterior horn. The direct 

 pyramidal tract represents a small remnant of 

 the fibres which descend in the cord without cross- 

 ing to the opposite side. They cannot as a rule 

 be traced further than the cervical region. Both of 

 these tracts are undoubtedly concerned in the trans- 

 mission of voluntary motor impulses from the 

 brain. Their degeneration always accompanies 

 paralysis of cerebral origin. 



The tracts which lie in immediate contact with 

 the graj matter, the anteroJateral mixed zone, 



