176 CONTENTS OF THE CHEST. 



great number of small cavities, in which the abundant white fluid of the gland is ii 

 part contained. From those cavities the fluid is transmitted into a genenl reservoir, 

 which forms a common connecting cavity, and is lined by a delicate membrane. Sir 

 Astley, and in this he is supported by Professor Miiller, believes that a peculiar albu- 

 minous fluid is conveyed by the thymus gland to the veins, through the medium of 

 the lymphatics. It has nothing to do with the formation of the blood, in the fcetua 

 or the child. 



These two eminent physiologists exert the better part of discretion, by declining 

 to give any hypothesis of its function beyond this, that it supplies the lymphatica 

 with an albuminous fluid. 



This gland continues to grow for some time after birth, and remains of considera- 

 ble size during the first year ; it then gradually diminishes, and, about the period of 

 puberty, usually disappears. It has, however, been found in a mare between five and 

 BIX years old. 



THE DIAPHRAGM. 



Bounding the thorax posteriorly, the base of the cone in the human subject, tne 

 interposed curtain between the thorax and the abdomen in the horse, is the diaphragm. 

 It is an irregular muscular expansion, proceeding from the inferior surface of 

 the lumbar vertebrae posteriorly and superiorly, adhering to the ribs on either 

 side, and extending obliquely forward and downward to the sternum ; or, rather 

 t is a flattened muscle arising from all these points, with its fibres all converg- 

 ing towards the centre, and terminating there in an expansion of tendinous substance. 

 It is lined anteriorly by the pleura or investing membrane of the thoracic cavity, and 

 posteriorly by the peritoneum or investing me^nbrane of the abdominal cavity. 



JLnatomy of the Diaphragm. In the short account which it is purposed to give of 

 the structure of the diaphragm, the description of Mr. Percivall will be closely fol- 

 lowed. " The diaphragm may be divided into the main circular muscle, with its 

 central tendinous expansion forming the lower part, and two appendices, or crura, 

 as they are called, from their peculiar shape, constituting its superior portion. The 

 fleshy origin of the grand muscle may be traced laterally and inferiorly, commencing 

 from the cartilage of the eighth rib anteriorly, and closely following the union of the 

 posterior ribs with their cartilages ; excepting, however, the two last. The attach- 

 ment is peculiarly strong; it is denticulated ; it encircles the whole of the lateral and 

 inferior part of the chest, as far as the sternum, where it is connected with the ensi- 

 form cartilage. Immediately under the loins are the appendices of the diaphragm, 

 commencing on the right side, from the inferior surfaces of the five first lumbar ver- 

 tebrae by strong tendons, which soon become muscular, and form a kind of pillar ; 

 and, on the left, proceeding from the two first lumbar vertebrae only, and from the 

 sides rather than the bodies of these vertebrae, and these also unite and form a shorter 

 pillar, or leg. The left crus or appendix is shorter than the right, that it may be 

 more out of the way of pressure from the left curvature of the stomach, which, with 

 the spleen, lies underneath. Opposite to the 17th dorsal vertebra, these two pillars 

 unite and form a thick mass of muscles, detached from the vertebrae, and leaving a 

 kind of pouch between them and the vertebrae. They not only unite, but they decus- 

 sate : their fibres mingle and again separate from each other, and then proceed onward 

 to the central tendinous expansion towards which the fibres from the circular muscle, 

 and the appendices, all converge.'* 



The diaphragm is the main agent, both in ordinary and extraordinary respiration 

 it assists also in the expulsion of the urine, and it is a most powerful auxiliary in 

 the act of parturition. In its quiescent state, it presents its convex surface towaids 

 the thorax, and its concave one towards the abdomen. The anterior convexity abuts 

 upon the lungs ; the posterior concavity is occupied by some of the abdominal viscera. 

 The effect of the action o/ this muscle, or the contraction of its fibres, is to lessen thfl 

 convexity towards the chest, and the concavity towards the abdomen : or perhaps, by 

 a powerful contraction, to cause it to present a plane surface either way. The abdo- 

 minal viscera that must be displaced in order to effect this, have considerable bulk ana 

 weight ; and when the stomach is distended with food, and the motion required from the 

 diaphragm jn rapid breathing is both quick and extensive, there needs some strong, firm, 

 elastic, substance to bear it. The forcible contact and violent pressure would bruise 

 nd otherwise iniure a mere muscular expansion; and therefor* "ve have this tendi- 



