ANATOMY AND PHYSIOLOGY OF THE SPINAL CORD. 399 



fluid. The latter is a serous liquid, which exists between 

 the cord and its serous covering, on both its superior and 

 inferior aspects, and is so abundant that the cord seems to 

 float loosely within it. It is usually more abundant in old 

 animals. Bernard remarks that, owing to the collection of 

 blood in the sinus during expiration, and to its withdrawal 

 through the suction power of the chest during inspiration, 

 this fluid is thrown up toward the brain in a regular suc- 

 cession of waves; and that a prolonged expiration will 

 accordingly lead to more or less vascular compression of 

 the brain. 



The spinal nerves 42 or 43 in number in the horse 

 arise each by two roots, a superior or sensory, and an 

 inferior or motor. The former, which is the larger, has, at 

 the point of exit from the spinal canal, a small ganglion, 

 in which, according to Leydig, the nerve-vesicles are 

 bipolar (having two prolongations). One prolongation of 

 each cell is continuous with an afferent fibre, whilst the 

 other is prolonged into one passing to the cord. Outside 

 the spinal canal, the roots meet to form mixed nerves, 

 which contain both sensory and motor filaments. 



A knowledge of the minute structure of the spinal cord, 

 and the mode in which its various parts are connected with 

 the nerves, is an essential preliminary to the proper under- 

 standing of the purpose it fulfils, in connection with the 

 various movements of the body. 



The antero-lateral columns of the spinal cord, according 

 to Van der Kolk, consist of white fibres running in great 

 part longitudinally and parallel to each other ; though, to 

 some extent, arranged in a transverse direction. The former 

 class seem to pass directly to the brain, and probably 

 convey volitions to the motor ganglionic cells in the anterior 

 hern of grey matter. Of the transverse fibres, some are 



