RESPIRATION. 



287 



the epithelium clothing the active capillary 

 segments forms a thicker layer (than in the 

 latter. This is the only true and ultimate 

 anatomical distinction between a gill and a 

 lung. 



The preceding general facts will form an 

 appropriate introduction to the study of 

 special details, on which it is proposed now to 

 enter. 



The Lancelet (Branchiostoma) occupies the 

 first grade in the vertebrate series. It exhi- 

 bits the branchial organs under the least com- 

 plex terms. A capacious branchial sac, the 

 dilated oesophagus, occupies the mid-portion 

 of the body. It communicates with the ex- 

 terior in front by means of a large uesophageal 

 opening, and behind by a branchial outlet and 

 a short intestinal canal. The parietes of the 

 stomach display special provisions for breath- 

 ing under the character of membranous du- 

 plications of the internal surface. These folds 

 are invested with a vibratile epithelium. In 

 this particular the branchiae of this fish ap- 

 proach those of the lower molluscs, and de- 

 part from those of all other fishes. A con- 

 venient arrangement of this subject will 

 consist in first studying the mucous membrane 

 of the branchire of fishes ; 2nd. the blood- 

 system ; and 3rd. the supporting frame-work. 

 Mucous membrane of the branchite. The 

 gills of fishes are lubricated and defended by 

 aglary viscid secretion : this is the product of 

 the epithelium. This latter, therefore, even 

 on these parts enacts a secernent office. On 

 the plane superficies of the leaflets, the epi- 

 thelium constitutes a single stratum, resting 

 immediately on the expanse of the rete mira- 

 bile. In this situation, as in all others, the 

 epithelial layer is supported by a limitary mem- 

 brane. From its extreme attenuation, how- 

 ever, it does not easily admit of separate defini- 

 tion. Neither cytoblasts, granules, or any other 

 immature particles, mingle with or underlie 

 this layer of adult epithelium. It is difficult 

 to conceive the mechanism of their renewal. 

 In the interior of its component cells, how- 

 ever, the eye clearly distinguishes a nucleus 

 and a few pellucid granules. It has already 

 been proved that these latter parts are almost 

 suppressed in the epithelium which lines the 

 active capillary segments of the true lung. 

 Of the branchial epithelium of fishes it may 

 be said that it unites the glandular to the me- 

 chanical function of the pulmonary epithelium, 

 that its office is exclusively mechanical. The 

 halitus of the air-cells is not a secreted pro- 

 duct. It is a transpiration. 



Viewed with reference to the principles of 

 exosmoses, a layer of amorphous granules, 

 the cytoblasts of future epithelia, interposed 

 between the blood vessels and the super- 

 ficial strata of adult cells, would obviously 

 render the partition to be traversed by the 

 gases engaged in respiration very inconve- 

 niently dense. The structure presented by the 

 epithelium of the branchiae implies, that in the 

 organs of aquatic respiration the blood is brought 

 less directly into relation with the external 

 medium than in those of atmospheric breathing. 



The cells investing the branchial capillaries 

 are not structureless, hyaline, flattened scales, 

 devoid of nucleus and granules, as though 

 the principle aimed at were merely the me- 

 chanical one of thinning to the extremes! 

 practicable limit all structures between the 

 blood and the outer medium. They constitute 

 irregularly oval bodies, carrying a nucleus, 

 and provided with a few pellucid molecules. 

 No cilia exist in the gills of fishes. In their 

 fixed condition, as in the cyprinoid families, 

 in their concealed situation, as in the shark 

 tribe, or their exposed and free position, as 

 exhibited in the higher osseous orders, forms 

 enough diversified, these organs are charac- 

 terised alike by the complete, and uniform 

 absence of vibratile epithelium. 



The Vascular System of the Branchies. In 

 fishes, the whole force of the heart and bulbous 

 aorta is expended upon the branchial circula- 

 tion. The power of the heart is materially 

 reinforced by the resilient structure which 

 composes the parietes of the aortic bulb. 

 This structure is remarkably contractile. By 

 all standard authorities on comparative ana- 

 tomy the muscular nature of the walls of the 

 bulbous aorta is admitted without question. 

 The interior of this " bulb" is strengthened 

 by cornice columnce. It is a fact of no common 

 interest that the colour of the muscular struc- 

 ture of the ventricle is higher or redder in 

 ground-fishes than in those species of which 

 the habitat is superficial in the water, and which 

 are gifted with the power of active locomotion. 

 These facts are indicative respectively of supe- 

 rior and inferior degrees of muscular irritability, 

 and well shown to conspire with other proofs 

 to determine a difference in amount between 

 the respiration of deep and surface-swimming 

 fishes. The valves which guard the proximal 

 and distal portions of the bulbus arteriosus 

 vary in situation, number and size, in different 

 families. The movement of contraction which 

 takes place in this vessel is not instantaneous 

 in duration, like that of the ventricle : it is 

 slow and vermicular. The pressure which 

 is thus exerted upon the column of the blood 

 is continuous. The near proximity of the 

 delicate capillary structures of the branchiae 

 to these powerful centres of force, demands 

 the graduated manner in which the aorta re- 

 acts upon the stream of the blood. This mode 

 of action also explains the purposes subserved 

 by the " valves," which in some instances 

 occupy an advanced situation in the vessel. 

 The abdominal aorta, which results from the 

 confluence of the branchial veins, differs con- 

 siderably in structure from the pre-branchial 

 division of the vessel: the parietes of the 

 former resemble those of a vein. No jmlsa- 

 tions are dctectible in the abdominal aorta of 

 fishes. The pulsatile movement of the blood, 

 derived from the systole of the ventricle, 

 ceases at the branchial capillaries. In fish, 

 therefore, arterial pulsations exist in no other 

 part of the circulating system than in that 

 limited segment which intervenes between 

 the branchial network and the cardiac ven- 

 tricle. Neither the head nor any other part 



