234 COMPARATIVE ANATOMY 



sacral plexuses, in which dorsal and ventral branches can usually 

 be recognised. 



In the brachial plexus, which from the Sauropsida onwards is 

 more sharply differentiated from the cervical plexus, the following 

 may be distinguished: (1) anterior (superior) thoracic nerves 

 (dorsalis scapulae and thoracicus lateralis of human anatomy); (2) 

 posterior (inferior) thoracic nerves (sidclavius, tlioracici anteriores) ; 

 (3) ventral (anterior) brachial nerves (mcdianus, with the musculo- 

 cutaneus, ulnaris, and cutaneus medius et internus) ; (4) dorsal 

 (posterior) brachial nerves (subscapulares, axillaris, and radialis). 



The lumbo-sacral plexus shows in general, and more particularly 

 in Mammals, much greater variations than does the brachial plexus. 

 The nerves arising from it are also arranged in a dorsal and a 

 ventral series, the larger ones being spoken of as the obturator, 

 crural, sciatic, and pudendic. The sciatic divides up in the hind- 

 limb into a tibial and &fibular nerve. The numerous individual 

 variations seen in the lumbo-sacral plexus (e.g. of Man) are due to 

 tne fact that the pelvic girdle has not yet acquired such a fixed 

 position with regard to the trunk as has the shoulder-girdle, and 

 shows a tendency to become shifted forwards. 1 



2. CEREBRAL NERVES. 



It is customary to distinguish the following twelve pairs of 

 cerebral nerves, and of these the eleventh have a distinct origin 

 only in the Amniota, and the twelfth are represented by the 

 anterior spinal nerves in certain Fishes and in all Amphibians : 



I. Olfactory. 



II. Optic. 



III. Oculomotor. 



IV. Pathetic or trochlear. 

 V. Trigeminal. 



VI. Abducent. 



VII. Facial. 



VIII. Auditory. 



IX. Glossopharyngeal. 



X. Vagus or pneumogastric. 



XI. Spinal accessory. 



XII. Hypoglossal, in close connection with which 

 are certain " spino-occipital " nerves. 



Of these, the purely sensory olfactory and optic nerves have 

 an isolated position as regards their mode of development from the 

 secondary and primary fore-brain respectively, and they are dealt 



1 In animals in which the extremities have disappeared, all traces of the 

 corresponding plexuses have also usually vanished : Snakes, however, still retain 

 remnants of them. 



