224 ANIMAL BIOLOGY. [Parti. 



and above the mandible, is seen a nerve passing to the integu- 

 ment of the upper jaw. It is the maxillary branch of the 

 trigeminal. 



5. Carefully remove the eye. To do so the optic nerve 

 supplying that organ must be severed. Passing along the 

 upper border of the orbital cavity is a nerve running forwards 

 to the nasal chamber. It is the orbito-nasal division of the tri- 

 geminal. It is not connected with the sense of smell. 



In the fowl and rabbit there are twelve pairs of cranial nerves. 

 Of these the first ten are essentially similar in distribution and 

 origin to those of the frog. The eleventh is a spinal nerve, which 

 (in the rabbit for instance) takes its origin a little posterior to 

 the fifth cervical nerve. It then passes forwards in the fork 

 between the two roots of the anterior spinal nerves, receiving 

 fibres from the cord en passant, and leaves the skull, together 

 with the ninth and tenth, to be distributed to some of the 

 muscles in the neck. The twelfth, or hypo-glossal, answers 

 to the first spinal of the frog, and has a similar distribu- 

 tion, but making its exit through the skull is reckoned as 

 cranial. 



Spinal Nerves. In the frog the first spinal nerve forms the 

 hypo-glossal. The second and third unite to form a brachial 

 plexus for the inner vation of the fore-limb. The fourth, fifth, and 

 sixth spinal nerves pass to the body-walls. The seventh, eighth, 

 and ninth form the lumbo-sacral or sciatic plexus, from which 

 nerves are given off to the hinder region of the body, but especi- 

 ally to the hind-limb, dividing into a small crural to the front of 

 the thigh, and a large sciatic to the back of the thigh. This 

 latter divides into the tibial, which ends on the lower surface of 

 the foot, and the peroneal which terminates on its dorsal surface. 

 The tenth spinal nerve leaves the neural canal by the coxygeal 

 foramen in the urostyle, and is distributed to the adjacent 

 parts. 



In the rabbit there is a brachial plexus formed by the union of 

 the fifth to the eighth cervical and first thoracic nerves. It gives 

 rise, as is shown in Fig. 72, to a circumflex (c/.), which dives into 



