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 lacunz and canaliculi being then in relation to the 
_ nearest vascular surface,—as is the case also with very thin 
| laminz of ordinary 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 
_ ang 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. x1); 
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 we 
Shall find that the special purpose of the whole apparatus of 
Organ c life, is, by providing materials for their nutrition and 
“Tenoyation, to build them up in the first instance, and then 
