TEETH.-MUSCLE AND NERVE. 
69 
the Rodentia), the enamel-prisms cross and interlace with one 
another, in such a manner as to prevent that separation 
which would readily occur if the direction of all of them 
were the same. Of all the tissues of the animal body, the 
Enamel is the most remarkable for the predominance of 
mineral ingredients; these amount to no fewer than 98 
parts in 100, leaving when removed only 2 per cent, of 
organic matter. The softer component of Teeth, known as 
the Cementum , or Crusta 'petrosa , possesses the essential 
characters of true bone; but when only a thin layer of it is 
present, we do not find it traversed by medullary canals, its 
system of lacunae and canaliculi being then in relation to the 
nearest vascular surface,—as is the case also with very thin 
laminae of ordiuary bone, such as we find in the scapula 
(blade-bone) of a Mouse. 
55. We come, lastly, to the two tissues which are of the 
highest importance in the Animal fabric, and to which all the 
rest are merely subsidiary; namely, the Muscular and the 
Nervous. It is through the instrumentality of these, that 
all the actions are performed which essentially constitute 
Animal life; for the nervous apparatus is the medium by 
which the consciousness of the individual is affected by what 
takes place around him, or within his own body, and by 
which, in his turn, he originates movements in his body, 
and through it in things external to it; whilst the muscles 
are, so to speak, the servants of the nerves, doing , with a 
force of their own, the work which the nerves direct. The 
relation between the two may be likened to that of the rider 
and his horse, or of the engine-driver and his locomotive ; for 
the nerves can put forth no motor power by themselves; 
whilst, on the other hand, the muscles (with certain excep¬ 
tions) remain inert except when stimulated to contract by the 
agency of the nerves. The muscles use the tendons and the 
framework of bones, joints, &c., for the mechanical appli¬ 
cation of their power, as will be shown hereafter (Chap, xil); 
but these parts of the fabric have not the slightest power of 
originating motion by themselves. Hence, all Animal Force 
takes its rise in one or other of these two tissues; and w^e 
shall find that the special purpose of the whole apparatus of 
Organic life, is, by providing materials for their nutrition and 
renovation, to build them up in the first instance, and then 
