1 86 ELEMENTARY PHYSIOLOGY 



LESSON XXXII. 

 THE SPINAL CORD. 



THE spinal cord is a long cylinder of nerve matter which is con- 

 tained in the spinal cavity formed by the vertebral column. It 

 extends from the medulla oblongata to the first lumbar vertebra^ 

 and measures about eighteen inches in length. It is nearly as 

 thick as the little finger. 



Like the brain, the spinal cord is closely invested by the pia 

 mater from which it receives its blood supply. Over this lies the 

 serous arachnoid membrane, and exterior to the arachnoid is a 

 continuation of the dura mater of the brain. 



The arachnoid membrane of the cord, unlike that of the brain, is not in 

 contact with the pia mater, but forms a loose bag around it. The dura mater, 

 also, does not lie close against the vertebrae as it does against the cranium, 

 but is separated from them by a layer of fatty tissue which forms a soft pro- 

 tecting covering, enabling the back to bend without injury to the cord. 



The student may often procure an uninjured piece of the cord from the 

 ' loin ' of mutton, the ' sirloin ' or the ' rib ' of beef. After noticing its general 

 character while fresh, he should soak it in spirit till sufficiently hard for 

 cutting sections. 



On making a transverse section of the spinal cord we notice 

 that the grey nerve-substance is in the interior, and is surrounded 

 by white matter. The cord is divided into right and left parts by 

 two deep depressions or fissures. The front or anterior fissure is 

 wider than the other, but not so deep. The posterior fissure is 

 not a true fissure, but simply a layer of thin connective tissue, 

 penetrating almost to the centre. In the centre of the cord is a 

 very small canal the central canal which can be seen only with 

 the aid of a microscope. 



Large nerves are given off from the spinal cord, right and left, 

 throughout its whole length. These are termed the spinal nerves. 

 In the cervical and dorsal regions they emerge in pairs, passing 

 out on each side through openings between the vertebrae. There 

 are thirty-one pairs of these nerves. At the lower end of the cord 

 the spinal nerves come off crowded together in the form of a 

 parallel bundle which is called the cauda equina (Lat. horse tail), 

 from its fancied resemblance to the tail of a horse. 



The grey matter of the spinal cord projects backward and 

 forward on each side, forming the posterior and anterior cornua 

 (Lat. cornu, a horn). From the posterior cornu a bundle of fibres 

 emerge, forming the posterior root of a spinal nerve. Fibres also 

 pass outward from the anterior cornu, giving rise to an anterior 



