THE NERVOUS SYSTEM IN GENERAL. 651 



But the effects of this radical transformation do not cease here. The 

 animal thus rendered apt to the nutritive movement acquires, beyond this 

 vegetative life, the common appanage of all organised beings, all the attributes 

 of what it has become habitual, after Bichat, to term animal life, that is, 

 sensibility, volition, instinct, and intelligence. 



The perceptive centre which receives the excitations developed at the 

 periphery of organs, or in their structure ; the excitatory centre which 

 induces motion in all the other tissues ; the seat of the instinctive and 

 intellectual faculties ; in short, does not the apparatus of innervation, thus 

 charged with the grandest physiological finality, present itself as a most 

 attractive study ? We will commence by giving a general and succinct idea 

 of its conformation, structure, properties, and functions, before undertaking 

 the special description of the different parts composing it. 



GENERAL CONFORMATION OF THE NERVOUS SYSTEM. 



The apparatus of innervation comprises a central and a peripheral 

 portion. 



The first represents a very elongated stalk lodged in the spinal canal, 

 and bulging at its anterior extremity, which occupies the cranial cavity. 

 This is named the cerebro-spinal axis or centre. 



The second consists of a double series of ramescent branches, which are 

 given off laterally from the central stalk, to be distributed to all parts of 

 the body ; these branches are the nerves. 



THE CEREBRO-SPINAL Axis. The stalk, or axis properly so called, 

 lodged in the spinal canal, forms the spinal marrow (or cord). It is a large 

 white cord, terminating in a point at its posterior extremity, and giving rise, 

 at each intervertebral foramen, to one of those nervous branches which, collec- 

 tively, represent the peripheral portion of the apparatus of innervation. 



The bulging extremity inclosed in the cranium, is named the enceplialon 

 (or brain). More complicated in its conformation than the spinal cord, this 

 portion is divided, as we will see, into four parts : 1, A white peduncle, the 

 continuation of the spinal cord ; 2, Three grey-coloured ovoid masses, one 

 of which is posterior, the other two being anterior, and placed symmetrically 

 side by side. This medullary prolongation emits, right and left, like the 

 cord itself, nervous branches destined almost exclusively for the head. 



THE NERVES. The nerves are in the form of fasciculated cords, and 

 make their exit from the orifices at the base of the cranium, or through the 

 intervertebral foramina, passing into all the organs by ramifying like 

 arteries, which they generally accompany. 



All the nerves have their origin from the medullary axis, or from its 

 encephalic prolongation, by radicles more or less apparent. They are 

 divided, according to the relative position of their point of emergence, into 

 two great categories ; the superior, arising from the corresponding face of 

 the spinal axis ; the others, inferior, escaping from the lower face : a distinc- 

 tion which is perfectly appreciable with regard to the cord itself, but which 

 is more difficult to establish in the encephalic peduncle, as it is less 

 distinct. 



At their emergence from the bony canals which give them passage, 

 the radicles of each nerve always unite into a thick common trunk. 



In the majority of cases, there enters into the composition of this trunk 

 the nerves or fibres of the two orders ; only a few nerves are composed of 

 fibres of the one kind, and these all belong to the brain. 



