THK ORGANS OF MOTION. 225 



157- 



The nerves consist of filaments formed of consecutive globules, which 

 are enclosed by delicate membranes, the nervous sheath (iieurilema), 

 These globules are originally very loosely connected, and the nervous 

 filament then appears as a delicate tube, which encloses a finely granu- 

 lated pappy mass. The first commencement of the nerves is found thus 

 formed, as well in the embryo of the superior animals, as also in all 

 the inferior ones ; and whilst the latter constantly retain this original 

 grade of organisation, the nervous cord in the former works itself on 

 in the progress of development to a firm filament. Several of such 

 little filaments form the thicker nervous thread, and several of these 

 the nervous cord. Where such threads or cords anastomose, meet, 

 or cross each other, the nervous mass distends and forms knots or 

 ganglions. That which we call the brain (cerebrum}-, which lies in 

 the head, is the largest and most perfect of these ganglia, and indeed 

 composed of various other smaller ones, and in its most perfect state of 

 organisation it is even furnished with internal cavities. It is there 

 first found where a head is first distinctly separated from the body. In 

 all animals without a head there is no brain, but their nerves originate 

 from a nervous ring encompassing the pharynx, which here represents 

 the central organ of the nervous system, whilst the brain, where it is 

 developed, gradually draws this ring to it. 



THIRD CHAPTER. 



OF THE ORGANS OF MOTION. 



158. 



THE organs of motion fall into two different sub-systems, namely, 

 the ACTIVE or muscles, and the PASSIVE. The passive organs of 

 motion are, according to the different groups, subject to great changes, 

 and only in the higher grades of animal development do they become a 



Q 



