182 
MR. YOUATT’s VETERINARY LECTURES. 
and character. It is still a dense and firm membrane, but it has 
acquired an elasticity which it had not before. 
The interposed Cellular Substance .—It is still the proper in¬ 
vesting membrane—the theca vertebralis ; but the cavity in 
which it is placed admits of very great change of direction and 
calibre, and is composed of a succession of joints, which, during 
the waking hours of the animal, are in almost continual motion. 
The spinal chord must not be affected or injured by these changes, 
therefore, as I have said, the dura mater, now the theca vertebra¬ 
lis, is detached from the parietes of the canal, and there is inter¬ 
posed between it, and the bony circle around it, a reticulated 
membrane, some of the cells of wdiich are filled with adipose 
matter, and others with serous fluid, and which, combined to¬ 
gether, will form the most effectual resistance to any occasional 
concussion or pressure. From the delicacy of thd membrane, so 
contrived that it shall adapt itself to every motion or change of 
form in the bony canal, it has a gelatinous appearance ; the fluid 
which it contains, however, has not the slightest.viscidity ; and 
this fluid is enclosed in separate cells, that the lesion of some 
may not endanger the general safety of the chord. 
The inner Membranes of the Chord .—This interposition of 
membranous and adipose and aqueous matter would seem to af¬ 
ford a tolerably secure protection to the spinal chord ; but, to 
render assurance doubly sure, there is a similar provision within 
the theca. The arachnoid coat , if such it be, is no longer a gos¬ 
samer secretory membrane floating between the dura and the pia 
mater, but it, too, takes on a reticulated, cancellated structure : 
it is composed of innumerable cells, containing, likewise, a limpid 
fluid, and performing the same office of defence; or, rather, com¬ 
pleting, and that beyond the reach of common injury, the de¬ 
fence of the spinal chord. 
The admirable contrivance of this interposed Cellular Substance . 
—Now, gentlemen, please to observe the effect of this. That por¬ 
tion of the spinal chord which assumes a somewhat vertical direc¬ 
tion in the neck is suspended by a thousand elastic springs, and 
surrounded by a yielding elastic substance whether the ani¬ 
mal is grazing or employed in our service, which, in all the exten¬ 
sive changes in the direction of the neck, afford a security against 
concussion with which no complication of springs of human con¬ 
trivance will bear the slightest comparison. You see this yet 
more evidently in the horizontal dorsal part of the spine; there 
observe the action of the elastic suspensory chords or springs 
above, and the downy yielding, yet strong, illacerable cushion 
beneath ; add to this the elasticity gained by the cartilaginous, 
ligamentous structure and mechanism of the joints of the spine, 
