AVES BIRDS. 607 



which is poured into the corresponding auricle by three large veins, 

 viz. one inferior and two superior vena? cavce. The contraction 

 of this auricle drives the blood into the right ventricle ; theauriculo- 

 ventricular opening being guarded by a broad fleshy valve, formed 

 by the muscular substance of the heart itself; and hence the venous 

 blood is forced through all the ramifications of the pulmonary arteries. 



The aerated blood is then returned from the lungs by two veins, 

 which pour it into the left auricle ; and the left ventricle, now en- 

 tirely appropriated to the systemic circulation, diffuses it through 

 the body ; thus all mixture of the venous and arterial fluids 

 being prevented, the system is supplied by the left side of the heart 

 with pure and highly vitalized blood. 



(675.) In the nervous system of Birds there is a very perceptible 

 improvement when compared with that of Reptiles, more especially 

 in the increased proportional developement of the cerebral hemi- 

 spheres : still, however, there are no convolutions seen upon the 

 surface of the cerebrum ; neither are those extensive communica- 

 tions between the lateral halves as yet developed, which in the 

 higher Mammalia assume such size and importance : the corpus 

 callosum and fornix are both wanting, a simple commissure being 

 still sufficient. Neither has the cerebellum in these animals as- 

 sumed its complete developement, presenting only the central por- 

 tion ; so that the pons Varolii^ or the great commissure, which in 

 Man unites the lateral cerebellic lobes, is of course deficient. The 

 olfactory and optic lobes are even here recognisable as distinct ele- 

 ments of the cerebral mass, and the origins of the nerves strictly 

 conform to the arrangement already described in the brain of Rep- 

 tiles. The rest of the cerebro-spinal axis presents no peculiarity 

 worthy of special notice ; and the general distribution of the cere- 

 bral and spinal nerves is so similar in all the Vertebrata, that it 

 would be useless again to describe them in this place. 



The sympathetic system in Birds is well developed, and its ar- 

 rangement differs in no essential particular from what is seen in the 

 human body ; the situation of the cervical ganglia is, however, 

 peculiar, inasmuch as they are lodged in the bony canal formed by 

 the transverse processes of the vertebrae of the neck for the recep- 

 tion of the vertebral artery, and are thus securely protected in spite 

 of the unusual length and slenderness which the neck not unfre- 

 quently exhibits. 



But if in the general arrangement of the nervous system of 

 the feathered races there is little to arrest our notice, we shall find 



