NERVOUS SYSTEM. 23 



destruction of the brain and spinal marrow, or the section of a 

 nerve is at once followed by a complete paralysis, either general 

 or local, and, very soon after this phenomenon occurs, it becomes 

 impossible to excite muscular contraction, by pricking or other- 

 wise stimulating the affected parts. In reptiles, on the contrary, 

 the faculty of moving under the influence of stimulation, is pre- 

 served under similar circumstances, for a very long time ; for 

 example, the tail of a lizard detached from the body, continues 

 to move for several hours, and we may often see a tortoise that 

 has been apparently dead for several days, move its limbs when 

 the muscles are stimulated by pricking them. We may conclude 

 that, in these animals, the division of the physiological labour and 

 localization of the different functions of the nervous system, are 

 not carried so far as in mammals and birds, and hence there is a 

 less intimate dependence of the different parts of the economy 

 upon each other. 



10. The encephalon of reptiles is but little developed ; the sur- 

 face of the brain is smooth and without convolutions. The two 

 hemispheres are oval, more or less elongated, and internally 

 hollowed by a single ventricle. The spinal marrow, compared 

 with the brain, is very much developed, and it is also remarked 

 that the nerves are larger, proportionally to the volume of the 

 central parts of the nervous system, than in the superior animals. 



11. Most reptiles have no special organ of touch, and the 

 nature of their integuments prevents their tactile sensibility from 

 being much developed. It is true, that in some of them the skin is 

 entirely naked, and the epidermis is scarcely distinct ; but, for the 

 most part, it is covered by a thick epidermic layer, composed of 

 horny or bony plates which are more or less hard. In reptiles 

 with a naked skin, and an epidermis of moderate consistence, it 

 is frequently detached and renewed ; and, in those animals where 

 it is most consistent, it becomes detached, at different seasons of 

 the year, to give place to a new epidermis ; sometimes this kind 

 of moulting is partial, or at least, the epidermis only falls off in 

 flakes ; but at other times it is entirely detached, and preserves 

 the form of the animal upon which it grew. Serpents shed their 

 coats in this way several times in the course of the year. 



12. The eyes are ordinarily small, but in their structure, are 

 like those of mammals and birds ; but there is rarely found in 

 them any prolongation or fold of the retina resembling thepccten. 



10. What is the general character of the Encephalon ? What is remark- 

 able in the nerves of Reptiles ? 



11. Have Reptiles an organ of touch ? Is the epidermis permanent in 

 Reptiles ? 



12. What is the general character of the eyes of Reptiles? Have they 

 any eye lids ? 



