16 ON THE STRUCTURE AND 



deprived of the power of uttering certain sounds ; but these 

 are produced by other means. 



The Organs of Sensation. 



The head contains all the parts that are most necessary 

 towards sensation; namely, the brain, the principal nerves, 

 and the organs of seeing, hearing, smelling, and tasting. Sen- 

 sation is generally believed to be an action commencing at 

 the extremity of the nervous system, and thence communi- 

 cated to the sensorium, or that part of the brain where the 

 nerves are united. The degree of sensibility in animals has, 

 by many writers, been believed to be greatest in those whose 

 brain bears the greatest proportion to the whole weight of 

 their bodies. This however can only hold good, at most, as 

 a general rule, since it has very many exceptions. Professor 

 Camper considers that it may be determined by measuring 

 the facial angle, or the angle formed by the junction of two 

 lines, one drawn from the forehead through the teeth, and 

 the other through the basis of the skull. The ancients seem 

 to have had some idea of this theory, since, in the statues of 

 their gods and heroes, they always made the facial angle 

 even greater than a right angle. 



In all the quadrupeds, the^rof/?, which is an homogeneous, 

 pulpy substance, contains precisely the same parts as in 

 men; and those parts are likewise disposed in a similar 

 order. Its exterior is of a reddish colour, from the numerous 

 blood vessels and nerves that are disposed on its surface ; 

 but within, where these do not penetrate, the colour is a 

 pure white. It is from the brain and the spinal marrow that 

 the nerves have their origin ; and these three form the com- 

 mon organ of sensation and volition. As it is impossible to 

 describe intelligibly the very delicate structure of the brain, 

 without either actual inspection, or accurate engravings of 



all 



