Prof. Allen Thomson on the Spinal Cord and Nerves. 349 



primitive nerve-tube which receives a deep colour from the carmine ; 

 and though, in many cases, these fibres are not more than ^ to ^th of 

 an inch in diameter, yet they are made so distinct that they can be 

 followed with the greatest ease, and their course and connections traced 

 in situations in which, from their miuuteness and complexity, it would 

 otherwise have been quite impossible to recognize them. 



The nerve-cells also, and particularly the multipolar cells of the gray 

 substance of the spinal marrow, are rendered more distinct by this 

 process ; and Dr. Thomson pointed out a number of instances, in the 

 preparations exhibited, in which it was probable that the radiating pro- 

 longations appeared to be spreading from various sides of these cells in 

 actual connection with the central filaments of nerve-fibres. 



In the preparations shown the vai'ious parts of the nerve-tubes, viz., 

 the external tubular sheath, the medullary substance, and the central 

 filament, were shown with great distinctness. 



Dr. Thomson gave some account of the views of recent inquirers as 

 to the course and direction of fibres in the different parts of the cord, 

 and their connection with the roots of the spinal nerves, — alluding more 

 particularly to the statements of Stilling, Schroder Van der Kolk, 

 Lockhart Clarke, and KoUiker. 



The preparations shown illustrated very fully the structm-e of the 

 central canal of the cord, with its epithelial lining ; upon which Dr. 

 Thomson remarked that, hke the ventricular cavities of the brain, it 

 was to be regai'ded as an involuted portion of the original external sur- 

 face of the germinal membrane, and that the epithelial lining might be 

 looked upon as indicative of this origin, and also as interesting in 

 connection with the peculiar structures which are now known to be 

 appended to the extremities of the nerves of special sense (olfactory 

 optic, and auditory), seeing that the extremities of these nerves are 

 probably rather expansions of circumscribed parts of the original central 

 nervous organ, than new developments in peripheral structures. 



In regard to the structure of the substance of the spinal cord, Dr. 

 Thomson stated the view which he and Dr. Reid were inclined to take 

 from their observations, that it is everywhere pervaded by a frame, 

 work of delicate fibrous or areolar substance, in which the nerve-cells 

 and nerve-fibres are placed, and through which run the blood-vessels 

 when present. 



In the cross section of the spinal cord this delicate framework pre- 

 sents itself most obviously, in the form of radiating processes, passing 

 from the gray substance internally towards the external surface, where 

 it is completed by a thin covering of the same tissue. It is well seen 

 also surrounding the larger multipolar cells of the anterior vesicular 



