THE RESPIRATORY APPARATUS IN BIRDS. 563 



around their spinal cord ; and observation justifies the correctness of this 

 prevision. The dura mater, whose capacity is so superior to the volume of the 

 marrow in Mammals, exactly measures the volume of that organ in Birds ; so 

 that there does not exist between the fibrous and nervous surfaces any space for 

 an accumulation of liquid : this anatomical fact is sufficient to demonstrate the 

 absence of sub-arachnoidean fluid in Birds. In denying the existence of this 

 fluid, it ought to be added that in this class of vertebrata, as in the preceding, 

 the spinal prolongation is covered by a triple envelope ; that in each, between the 

 pia mater and dura mater, is found a thin transparent membrane, which is 

 lubricated by a serous fluid ; but here this fluid does not collect, it only moistens 

 the arachnoid membrane. 



"Considered in their dorsal portion, the prolongations springing from the 

 cervical reservoirs offer an entirely different arrangement to that already noticed. 

 The interspinal current, having entered the thorax, terminates by passing into 

 the first dorsal vertebra ; after coursing through every part of this vertebra, it 

 escapes by a lateral orifice into a small sac situated between the two first ribs, at 

 the origin of the first dorsal nerve ; from this sac, it passes into the second 

 vertebra by an opening placed on its antero-lateral part, then it flows back from 

 this into a new air-sac developed between the second and third ribs ; and passing 

 in the same manner into the third vertebra to sweep through a third intercostal 

 sac, it arrives nearer and nearer the last dorsal vertebra. In their dorsal portion, 

 the prolongations emanating from the cervical reservoirs thus form two currents, 

 though these are constituted alternately by the vertebrse and the small air-sacs 

 placed on their lateral aspect. At the same time that these sacs receive the air 

 from the vertebras preceding them, and transmit it to those which follow, they 

 communicate it to all the vertebral ribs. 



" In no order of Birds do the terial currents leaving the cervical reservoirs 

 communicate with those which circulate in the cranium. Liquids injected either 

 by the serial portion of the vertebral canal or the lateral prolongations of the 

 neck, never enter the bones of that cavity. Thinking that the injection might 

 perhaps penetrate if passed in the opposite direction, we have perforated the 

 bones of the cranium, and to the aperture have adapted the extremity of a steel 

 syringe filled with mercury ; but the metal did not reach the aerial prolongations 

 of the neck. From this double experiment, we concluded that the cranial bones 

 have no communication with the respiratory apparatus." 



3. Anterior diapkragmatic rese^-voirs (Fig. 337, 3). — " Placed between the 

 two diaphragms, they correspond : in front, to the thoracic reservoirs, against 

 which they stand ; behind, to the posterior diaphragmatic reservoirs ; outwardly, 

 to the ribs and intercostal muscles ; inwardly, to the thoraco-abdominal diaphragm 

 and oesophagus ; below, to the most distant part of the thoracic reservoir ; above, 

 to the pulmonary diaphragm, which separates them from the corresponding 

 lung. These air-sacs communicate with the lungs by a circular opening, 

 which originates from the great posterior diaphragmatic bronchus ; there is 

 often a second opening of communication beyond the embouchure of the trunk ; 

 this reservoir is the only one which receives air from the lung by a double 

 orifice." 



4. Posterior diaphragmatic reservoirs (Fig. 337, 4). — " Oval-shaped like the 

 preceding, and situated like them in the interval which separates the two 

 diaphragms, these air-sacs are in contact, by their anterior part, with the 

 anterior diaphragmatic reservoirs, with which they form a vertical and trans- 



