3 36 ELEMENTARY ANATOMY. [LESS. 



in what is called the nervus impar or central ligament, which 

 is a delicate filament passing down in the middle of the cauda 

 equina to the bottom of the sacrum or to the coccyx. It is 

 composed almost entirely of the pia mater which originally 

 enveloped the primitively co-extended spinal marrow. 



Two enlargements are found in the spinal marrow : one 

 (called the cervical) at about from the third cervical ver- 

 tebra to the first dorsal ; the other (called the lumbar) at 

 about the last lumbar vertebras. These do not exist at first : 

 they are related to the nerves of the limbs. 



A minute modification of tissue, the canalis centralis, con- 

 tinues on downwards, from the fourth ventricle of the brain, 

 along the middle of the spinal marrow. In the earliest 

 stages of life this is a very conspicuous longitudinal cavity, 

 being in fact the canal formed by the elevation and closing 

 over of the walls of the embryonic primitive groove. 



19. In that the spinal marrow of man is medianly grooved 

 both on its ventral and on its dorsal surface, it follows the 

 rule of the Vertebrate sub-kingdom ; and in that it is con- 

 tinuous with the medulla oblongata, it agrees with all 

 Vertebrates except the Lancelet. 



As to the extension of the spinal marrow in the neural 

 canal, man's embryonic condition agrees with that which is 

 permanent in lower forms such as Fishes. In that very class, 

 however, e.g. in Diodon, the spinal marrow may be much 

 more shortened, and the cauda equina very much longer 

 relatively than in man, while in his own class (as e.g. in the 

 Hedgehog) these characters may be more marked than they 

 are in him, and a conspicuous cauda equina is found in Frogs 

 and Tortoises. 



The posterior end of the spinal marrow may present a 

 slight enlargement, as e.g. in the Cod. 



The canalis centralis is much more conspicuous in lower 

 forms, and in them persists plainly throughout life as a distinct 

 longitudinal cavity. A singular enlargement of this canal 

 may take place towards the lower end of the spinal marrow, 

 as in Birds, where a sort of inferior repetition of the fourth 

 ventricle (called the sinus rhomboidalis] exists in the situation 

 of the lumbar spinal enlargement. 



The cervical enlargement may alone exist, as in Cetaceans ; 

 while, on the contrary, the cervical and lumbar enlargements 

 may be both so far more marked than in man that the 

 intervening part of the spinal marrow is reduced to a very 

 slender condition, as is the case in the Tortoises. 



