478 THE RESPIRATORY APPARATUS. 



point nave led us to the conclusion that all the canaliculi open into one another, and 

 by this anastomosis constitute an extricable plexus whose various parts communicate 

 with each other." 



Finally, it may be mentioned that " the walls of the pulmonary canaliculi, examined 

 microscopically, appear to be covered internally with irregular septa which circumscribe 

 the areolae, and give them a cellular aspect." 



THE Am-SACS. 1 " In birds, the pulmonary mucous membrane is continued, at the 

 level of the orifices in the lung, into the utriculiform cavities which are developed 

 between the walls of the thorax and the abdomen on the one side, and the thoracic and 

 abdominal viscera on the other. These air-reservoirs exist in all the vertebrata of the 

 second class. In all, they are situated at the periphery of the viscera in the trunk, in 

 such a manner that Cams has justly observed that the lungs of birds inclose nil the 

 other viscera ; so that when they are distended by the entrance of air, they generally 

 depress these viscera by pushing them towards the median plane. In all, they are 

 independent of each other, and freely communicate either with the lung by a single 

 aperture, or with the bones by one or more openings. Lastly, in all they are nine in 

 number. 



" These reservoirs are the thoracic sac, situated at the anterior part of the thorax 

 two cervical reservoirs, situated at the base of the neck ; tivo anterior diaphragmatic 

 reservoirs, placed between the two diaphragms ; two posterior diaphragmatic reservoirs, 

 also between these two diaphragms, but behind the preceding and, lastly, two 

 abdominal reservoirs, placed against the superior wall of the abdomen. Of these nine 

 reservoirs, the first only is single and symmetrical ; the others are pairs, and similarly 

 arranged on each side of the median plane. 



" The thoracic and cervical reservoirs are situated beneath, and in front of, the lungs ; 

 the abdominal reservoirs lie behind these organs, and the four diaphragmatic sacs at 

 their inferior part and between the preceding; hence the denomination of middle 

 reservoirs sometimes applied to the latter, in opposition to the first, which are named the 

 anterior reservoirs, ami to the second, called the posterior reservoirs." 



EXTEIINAL CONFORMATION OF THE RESERVOIRS. 1. Thoracic reservoir (Fig. 246, 2). 

 " It is situated above the clavicles and the inter-clavicular space, in the cavity of the 

 thorax, which it extends beyond on each side to the roots of the wings, around the 

 articulation of the shoulder. It is related with : above, the trachea and oesophagus on 

 the middle plane, the lungs and the origin of the cervical reservoirs on the lateral parts ; 

 below, with the sternum, the clavicles, and the interclavicular aponeurosis; behind, 

 with the heart and anterior diaphragmatic reservoirs, beneath which it is prolonged by 

 forming on each a long point ; in front, with the integuments of the neck, which it raises 

 into a hemisphere in Palmipeds, but which is angularly depressed in other classes ; on 

 the sides, with the sternal ribs, the two clavicles, and the membrane uniting them. 



" The prolongations which arise from the lateral parts of these reservoirs, and cross 

 the walls of the thorax to pass around the articulation of the shoulder, are three in 

 number, and may be distinguished into inferior or subpectoral, superior or subscapular, 

 and middle or humeral. 



" The subpectorcd prolongation (Fig. 246, d) issues from the thoracic reservoir by an 

 orifice situated behind the posterior clavicle, and passes beneath the tendon of the great 

 pectoral muscle, where it spreads out as a lenticular cavity. The relations it contracts 

 with that muscle are remarkable : in birds, still more than in Man and a great number 

 of quadrupeds, the tendon of the great pectoral is formed of two parts, one direct, the 

 other reflet-ted ; it is between thes two portions that this small air-sac is insinuated, and 

 where it forms a very firm connection with them ; the effect of which is, that at the 

 moment the great pectoral muscle contracts, it dilates the subjacent cell and draws into 

 it a greater quantity of air. 



" The subscapular and humeral prolongation communicate with the principal reservoir 

 by a common opening placed behind the small adductor muscle of the humerus. After 

 leaving this orifice, the subscapular sac spreads under the scapular and subscapular 

 muscle, which it separates from the ribs and corresponding intercostal muscles , it is 

 developed more particularly in a longitudinal direction. 



" The humeral prolongation occupies the axilla ; it is smaller than the preceding, of a 

 pyramidal form, and opens by its summit into an infundibular fossa, which leads to the 

 canal of the humerus. 



"The thoracic reservoir differs from all the others by the extremely numerous 



1 What is said relating to these air-sacs is taken from the Memoir of M. Sappey 

 Recherches Sur 1'Appareil Respiratoire des Oiseaux,' Paris, 1847. 



