MEMBRANES OF THE SPINAL MARROW. 155 



After reaching the fourth dorsal vertebra it ascends obliquely to the 

 left side, and entcrsi^ the junction^ of the left subclavian and the 

 internal jugular veins. 



Right thoracic cluct}^ — It is a short trunk, formed by the union 

 of the lymphatics of the right side of the head and neck, and 

 right upper extremity. It terminates at the junction of the right 

 subclavian in the right internal jugular vein. 



SECTION VI. 

 NERVOUS SYSTEM. 



The principal divisions of the nervous system are the Brain, 

 Spinal Marrow, and Nerves. 



The tissue of this system is included in membranes or sheaths, 

 and consists o^ two differently coloured pulpy materials ; one of 

 which is the ivhite or medullary^ and the other the gray or cineritious. 



The white or ^medullary structure is fibrous ; these fibres are 

 looped ; in the centre of each is a stripe called the cylinder axis of 

 Furkinje. 



The gray is more vascular and is globular in its structure. 

 These globules are kidney-shaped, and contain a vesicular nucleus 

 with a necleolus. They are soft and of a yellow or brownish colour. 



All ganglia and nervous centres consist of a mixture of white 

 fibres and gray globules. 



The sheath of the nerves is called the neurilerama, and the inter- 

 nal material neurine. 



An anastomosis is the interchange of fasciculi between two trunks ; 

 each single fasciculus remaining unaltered by, although in contact 

 with, another. 



A plexus is a combination of anastomoses into a network. 



Pacinian corpuscles, are small elliptical bodies, found attached to 

 the ulnar and digital nerves. They are of a dull white colour, or 

 opalescent, and about two-thirds of a line in length. 



MEMBRANES OF THE SPINAL MARROW. 



The spinal marrow has three coverings. The dura mater is a 

 white fibrous membrane, and is external. It is continuous with 

 that of the brain, and investing the spinal marrow loosely terminates 

 at its inferior extremity in a pointed cul-de-sac. It does not adhere 

 closely to the bony canal, except at the intervertebral foramina, 

 where it affords sheaths for the nerve's. Between it and the walls 

 of the bony canal is a quantity of loose cellular tissue infiltrated with 

 fat and serum. 



The arachnoid is the middle covering. This is a serous mem- 

 brane, investing the spinal marrow, and lining the inner surface of 

 the dura mater, to which it gives a smooth, polished appearance. It 

 is extremely thin and transparent, and destitute of red blood-vessels. 



