524 COMPARATIVE NEUROLOGY. 



In the posterior limb the dorsal nerves of the digits are formed by the three 

 terminal branches of the musculo-cutaneous nerve. The external and internal 

 plantar nerves supply digital branches, the former to all the digits except the 

 internal branch of the fourth. 



RODENTIA. 



The brain of the rabbit is more lowly organised than that of any other 

 domesticated mammal. The convolutions cannot be said really to exist ; and 

 the Sylvian fissure is merely indicated. No portion of the cerebellum is 

 covered by the cerebrum when in situ. The olfactory lobes are very large, 

 and the testes are smaller than the nates. 



Although in this animal the tail is very short, the spinal cord extends beyond 

 the sacrum. A singular nerve is formed by a branch from the superior laryn- 

 geal, and usually one from the pneumogastric, and proceeds down the neck 

 with the sympathetic trunk, entering the thorax to reach the substance of the 

 heart ; it receives twigs from the first thoracic ganglion. The name "depressor 

 nerve of the circulation" has been given to this nerve by its discoverers, 

 MM. Cyon. 



AVES. 



In the nervous system of birds the chief point of interest is the brain, which 

 is less highly organised than that of the mammalia. The cerebrum is pointed 

 anteriorly, the hemispheres being separated by a shallow longitudinal fissure. 

 There is a very slight indication of the fissure of Sylvius, but the convolutions, 

 corpus callosum, and septum lucidum are absent. Two rounded bodies, the 

 optic lobes, occupy the position of the mammalian greater lobes. The lateral 

 lobes of the cerebellum are very rudimentary, and the pons Varolii is wanting. 



The cranial nerves are, in number, twelve pairs, and they resemble those of 

 the mammal in their origins, and, in a general sense, in their distribution 

 also. 



In birds the spinal cord extends into the coccygeal region. The nerves of 

 the brachial plexus are divisible in the wing into two groups ; a posterior 

 which represents the brachial, internal cutaneous, and radial ; and an anterior 

 representing the cubital, median, and musculo-cutaneous. 



The lumbo-sacral plexus likewise consists of two portions, the anterior 

 supplying the fascia lata, crural, internal saphenic, and obturator nerves ; 

 while the posterior gives off the gluteal, great sciatic, and popliteal nerves. 



