78 



ANATOMY OF VERTEBRATES. 



different mammals: it is greatest in the Cetacea, Sirenia and 

 Seal-tribe, the space between the myelon and neural arches being 

 occupied by blood vessels, which, in those aquatic orders, are 

 chiefly arterial plexuses. In land-mammals and Man the veins pre- 



Communication of the ' perineurar sinus with 

 the veins of vertebral centrum, vii". 



Transverse section of dorsal vertebra and 

 contents of its neural canal, xix". 



dominate, having more or less of the character of sinuses, as shown 

 in the section of the lumbar vertebra, fig. 42, where the communi- 

 cation of the f perineural ' veins, d, with those of the tissue of the 

 vertebral centrum, is shown. But the most constant fluid exter- 

 nal to the myelon is that which has been called 6 cerebro-spinal.' 

 In the dorsal region of the neural canal, in Man, the position of 

 this fluid is shown in fig. 43, where c is the myelon, with its pia 

 mater and arachnoid, in the dorsal or posterior septum, n the 

 nerve-roots, and s s the sub- or ent-arachnoid space. The use of 

 the uniform support and defence afforded by the interposition of 

 this fluid between the myelon and the hard walls of the neural 

 canal is obvious. 1 



The arachnoid is disposed about the myelon, as about the brain, 

 after the manner of the serous membranes ; it consists of an 

 exterior or ( parietal layer ' reflected upon the myelon to form the 



internal or ( myelonal ' layer. If a section be 

 made through a pair of nerve-roots, those e.g. 

 of the fifth cervical, fig. 44, the arachnoid is 

 seen to be continued as a loose sheath, about 

 the inter-neural part of the root, n n, and is 

 reflected so as to form small culs-de-sac, at 

 the orifices of emero-eiice. 



O 



In Man the myelon is loosely invested by 

 the ' dura mater,' to which it is attached by 



In which (he effects of the removal of this fluid in the Dog are described. 



44 



Transverse section of the 

 myelon and its membranes 

 across the roots of the 

 fifth cervical nerves. 



1 XIX". 



