TEETH. MUSCLE AND NERVE. 69 



the Eodentia), the enamel-prisms cross and interlace with one 

 another, iii 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 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 

 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, xn.); 

 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 

 Organic life, is, by providing materials for their nutrition and 

 renovation, to build them up in the first instance, and then 



