8 ' INTRODUCTORY LECTURE. 



consists of dorsal and ventral columns. "Whilst the Invertebrata 

 manifest a general tendency to development in breadth, the Ver- 

 tebrata rather gain in height by tliis doubling or repetition of 

 parts in the vertical or dorso-ventral direction : and in this we may 

 discern the tendency to rise above the surface of the earth, until in 

 man the entire body is uplifted ; and what is below and above in all 

 other Vertebrata, in him becomes before and behind. 



The general external integument in the Vertebrata is rarely bur- 

 thened and clogged by large and massive calcareous plates, but is 

 usually defended by light, and sometimes exquisitely organised and 

 singularly complex developments of the epidermal covering ; modified 

 according to the spheres of existence, the habitual temperature and 

 movements, and therefore eminently characteristic of the different 

 classes of Vertebrated animals. 



The actions of the unusually developed nervous element, — Avhether 

 the vibrating filament conveys to the sentient centre impressions 

 from without, or, obedient to the inward intelligence, imparts from 

 within the stimulus of volition to the moving fibre, — are essentially 

 productive of change. It is most probable that the same nervous 

 fibre is not equally fit for two successive actions ; but needs, after 

 each, a certain amount of restoration. The same may be predicated 

 of the action of the muscular fibre ; viz., that some change, no matter 

 how small, but to that extent unfitting it for the due repetition of 

 the act, is the consequence of its stimulated contraction : and thus 

 the continued existence of the living animal requires the presence of 

 organs of renewal and repair in intimate, but harmonious combination, 

 with those of sensation and motion. 



The raw material of this restoration is derived from without : the 

 alimentary canal, in which the conversion and animalisation of the 

 food take place, is provided, in the Vertebrata, with two apertures, 

 an entry or mouth {os), and an excremental outlet (as). The jaws 

 (_/) are two in number, and placed one below or behind the other, 

 working vertically or in the axis of trunk ; the principal part of 

 the alimentary canal is contained in an abdominal cavity, and is sup- 

 ported by a reflection of the serous membrane upon the walls of tliat 

 cavity ; and the canal is divided into oesophagus (ce), stomach (g), 

 and intestine (^). All Vertebrata have a liver (/) which is usually 

 a very complicated gland, with a special venous or portal system of 

 vessels ; and the biliary secretion is conveyed into the conunence- 

 ment of the intestine. The pancreas {p), which in most Vertebrata 

 presents the form of a compact and conglomerate gland, adds its se- 

 cretion to the bile in the duodenum. The spleen {s), a cellulo- 

 vascular ganglion, or gland without a duct, makes its first appearance 

 coincidently with that of the po)'tal vein, and manifests a prc)gressive 



