STRUCTURE OF THE SPINAL CORD. 605 



constitution of the Spinal Cord here advocated by the Pathological researches 

 of Dr. Luclwig Tiirck ;' who has shown that certain lesions of the Encephalon 

 produce a degeneration of nerve-tissue in particular tracts, which may be 

 traced continuously down the Spinal Cord usually in the anterior column of 

 the side affected, and in the lateral column of the opposite side ; whilst, on 

 the other hand, local lesions of the Spinal Cord, as from caries of the vertebrae, 

 or from the pressure of tumors, produce a like degeneration in certain tracts 

 of the posterior columns, and sometimes also of the lateral columns, ascending 

 towards the Encephalon. Thus it appears that the posterior fasciculi are 

 liable to this secondary degeneration in the centripetal direction only, and the 

 anterior in the centrifugal direction only ; the degeneration taking place, in 

 each case, in the direction in which they ordinarily transmit nerve-force. 

 The mixed endowments of the lateral columns are also indicated by these 

 phenomena. 



486. We are not required, however, by the adoption of this view of the 

 constitution of the Spinal Cord, to regard its Cephalic tibres as of a different 

 order from those which pass from one of its own segments to another; fur, con- 

 sidering the whole of the Crauio-spinal axis as one series of centres, receiving 

 the terminations of all the nerves, its longitudinal fibres are equally commis- 

 sural, whether they establish the connection between the nerve-roots and 

 vesicular matter of two adjacent segments, or whether they bring into the 

 same structural relation the parts which are furthest removed in position. 

 And thus we may regard all impressions upon the afferent nerves as first 

 operating upon it (affecting the consciousness, or not, according as they 

 reach the Sensory Ganglia, or are arrested in their progress thither); and all 

 motor impulses, whether purely reflex, or originating in volitional direction 

 or emotional excitement, as issuing immediately from it through the motor 

 trunks. If such be the case, it does not seem at all improbable that there 

 should be a difference in different tribes of animals, as to the proportion, 

 of fibres which have their centres in the Spinal Cord and in the Seusorial, 

 centres respectively; for in those whose ordinary movements of progres- 

 sion, etc., are independent of sensation, being performed through the reflex, 

 action of the spinal cord, it might be expected that the chief connection of 

 the spinal nerve should be with its own gauglionic substance, and that the 

 bulk of the fibrous columns should be composed of commissural fibres 

 resembling those which intervene between the separate portions of the gan- 

 glionic tract of the ventral cord of Articulata; whilst in like manner it 

 might be anticipated that in man, so large a part of whose movements are 

 performed in obedience to a mental stimulus and under the guidance of sen- 

 sation, the longitudinal strands should be chiefly composed of fibres that 

 directly connect the Sensorial centres with the roots of the spinal nerves. 

 Such a difference would appear, from the comparative researches of MM. 

 Volkmanu and Kolliker, to exist between the structure of the Spinal Cord 

 of the Horse and that of Man. 



487. The Medulla Oblongata* or cranial prolongation of the Spinal Cord, 



1 See his Memoir, Ueber secondlire Erkrankung einzelner Riiekensmarksstrahge 

 und ihrer Fortsetzungen zum Gehmie, in Denkschriften der Kaiserlichen Akademie 

 der Wissensehaften, Wien, 1851 ; also Zeitschrift der Gesell. der Aertze zu Wien,. 

 Band ix, Heft 10. 



2 For good accounts of the anatomy of the Medulla Oblongata, see Dr. John Reid, 

 in Edinb. Med. and Surg. Jour., 1841, Mr. L. Clarke in tin- Phil. Trims., 18-38, vol. 

 i, and 1868, Pt. i, and Dr. John Dean, 8vo , 1863; Schroder v. cl. Kolk, Syd. S"c. 

 Trans. ; Meynert, Art. Brain of Mummalia in Strieker's Hum. and Comp. Histology, 

 Syd. Soc. Trans., vol. ii, 1872, p. 867. 



