30 HIGH SCHOOL ZOOLOGY. 



tebrates, where the limhs are of greater importance in connec- 

 tion with the support of the body and with locomotion. 



33. Nervous System. — Here we recognize two constitu- 

 ent parts ; the central nervous system, and the nerves which 

 course from it to their endings in the various organs of the 

 body, and which indeed are developed as outgi'owths from it. 

 The nerves either caiTy im})ulses from the central nervous 

 system to the inu.scles, glands &,c., or they transmit impulses to 

 it from the various sense-organs ; they are thus either efferent 

 or afferent in function. They originate from both parts of the 

 nervous system — the brain and spinal cord, and are distinguished 

 therefore as cranial and spinal nerves. 



34. In our study of the skeleton we have seen that the brain 

 and S2:)inal cord are protected in bony canals, perforated for 

 the escai)e of the nerves. The canals are not entirely filled 

 up by these organs, for certain membranes are present which 

 assist in the })rotection and nutrition of them. The cranial 

 cavity for exam})le is much larger than necessary to hold the 

 brain, and the interspace is filled np with fatty matter which 

 it is necessary to remove before the brain is exposed. Wlieu 

 this is done, it is seen to have the form represented in Fig. 1 1 

 composed of alternately dilated and constricted parts. In front 

 are the olfactory bulbs close against the nasal sacs, connecting 

 these with the rest of the brain are the olfactory tracts, and 

 then come in order the cerebral hemispheres, the epiphysis 

 projecting from the concealed thalamic region, the optic lobes, 

 the cerebellum partly covering these and the medulla oblon- 

 gata, with two pairs of secondary swellings npon it. From 

 the under surface will be seen the cerebral hemispheres, with 

 tke optic nerves crossing l)ehind them after their descent from 

 the optic lobes, the thalamic region in the form of the hypo- 

 physis and inferior lobes, and the medulla oblongata with the 

 other cranial nerves springing from it. 



