146 MAMMALIA. 



the trapezius muscle. In the ass, it gives a large branch to the sterno-mastoid muscle, 

 it communicates with the first and second cervical nerves and terminates in the 

 trapezius muscle. In the porpoise it is small, it begins from the spinal cord near the 

 posterior root of the first cervical nerve, it acquires more fibrils as it ascends, it passes 

 out to the sterno-mastoid and trapezius muscles. 



SPINAL CORD. The membranes of the spinal cord are the same as those of 

 the brain, with the addition of the denticulated ligament ; they correspond with the 

 same in man. The external and internal appearance of the cord is very similar to that 

 in man, but is larger or smaller in different regions, according to the motions of the 

 spine, and the quantity and size of the nerves to be produced. Its structure is in 

 many respects like that of the brain, being composed of grey and white matter. In 

 its natural state, when divided longitudinally, it appears almost without any definite 

 character ; but an horizontal section shows the grey within the white, forming four 

 cornuated processes. To the naked eye the white and grey appear defined, but in a 

 dry thin preparation the grey is seen to radiate amongst the white. When hardened 

 in spirits its surface may be separated into irregular longitudinal fibres, which, when 

 dried, appear to be composed of meshes similar to those of the brain. The internal 

 position of the grey favours the safety of the delicate roots of nerves originating from 

 it, and allows an uninterrupted supply of blood ; and the white, aided by the pia mater, 

 furnishes them with a greater firmness and security externally. The nervous roots, in 

 radiating from the grey matter towards the circumference, appear as fine wavy threads 

 communicating with each other, and with the meshes of white, forming longitudinal 

 tracts. There is a more or less thick layer of roots continued from one end of the cord 

 to the other. Each root is defined by an accompanying artery ; the roots emerged 

 from the white matter appear beneath the pia mater in coarser wavy lines just before 

 their collection into fasciculi for forming externally the fibrils previously to their 

 combination into separate nerves. 



If a portion of the injected cord be dried and magnified, and the multiplicity of 

 minute vessels noted, it is difficult to determine how they are with such regularity 

 replenished with blood throughout the entire cord during the frequent and varied 



