LESSON 87.] NEKVOUS SYSTEM IN MAMMALIA AND MAN. 281 



So, too, "blood-vessels are formed, and the blood freely circulated 

 long before the existence of a heart. This may be beautifully seen 

 in the ova of Fishes, and especially in the egg of the domestic fowl, 

 on the fourth day of incubation. 



But the character of this circulation is more nearly allied to the 

 vegetable than to the animal kingdom, and closely resembles the 

 circulation, or rotation, as it is called, so beautifully seen in the 

 hairs of many plants, Tradescantia, the cells of Chara, Nitella, Va- 

 lisneria, Frog bit, &c. 



Still, in his early development, man maintains his supremacy, 

 for, during these progressive changes which take place in secret 

 unseen his external form is always human ! 



LESSON LXXXYII. 



NERVOUS SYSTEM IN MAMMALIA AND MAN, CONTINUED. 



1260. The spinal chord of the adult man (Fig. 896, 5, i) is 

 smaller, compared with the cerebral mass contained within the 

 cranium, than in other mammalia, and short, from the want of caudal 

 prolongation of the trunk ; its posterior and middle enlargements 

 (c, d) are conspicuous, and of a lengthened form, from the magnitude 

 and number of the nerves which proceed from them to the sacral 

 extremity ; the cauda equina (a, b) is of great length ; the motor 

 roots (nerves of motion, Z, Z) and anterior columns are smaller than 

 the sensitive, and the ganglia of the posterior or sensitive roots (k) 

 of the spinal nerves are here larger than in other mammalia. 



1261. The medulla oblongata, though comparatively small, has 

 its component fasciculi most deeply marked, and the quantity of 

 internal ganglionic matter in the course forwards of these white 

 fibrous fasciculi corresponds with their great development in the 

 human cerebral and cerebellic hemispheres, where the convolutions 

 (g) arid laminae (h, i) surpass in number and depth those of almost 

 all inferior animals. 



1262. It now only remains to ascertain the microscopical charac- 

 ters which distinguish a nerve and a ganglion in all animals 3 * and 

 these characters are found to be remarkably constant. 



1263. The fine, white chord, called a nerve, when placed under 

 the microscope, is found to consist of three tissues ; of these, the 



