232 THE CONTENTS OF THE CHEST. 



structure, and possible function of the thymus gland. It presents, on being cut 

 Into, a great number of small cavities, in which the abundant white fluid of the 

 gland is in part contained. From those cavities the fluid is transmitted into a 

 general 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 albuminous 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 foetus 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 lymphatics with an albuminous fluid. 



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

 siderable 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 six years old. 



THE DIAPHRAGM. 



Bounding the thorax posteriorly, — the base of the cone in the human sub- 

 ject, — the 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 it is a flattened muscle arising from all these points, with 

 its fibres all converging towards the centre, and terminating there in an expan- 

 sion of tendinous substance. It is lined anteriorly by the pleura or investing 

 membrane of the thoracic cavity, and posteriorly by the peritoneum or invest- 

 ing membrane of the abdominal cavity. 



Anatomy 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 followed. " 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 interiorly, commencing from the cartilage of the eighth rib ante- 

 riorly, and closely following the union of the posterior ribs with their cartilages; 

 excepting, however, the two last. The attachment 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 ensiform cartilage. Imme- 

 diately under the loins are the appendices of the diaphragm, commencing on 

 the right side, from the inferior surfaces of the five first lumbar vertebras, 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 vertebras, and these also unite and form 

 a shorter pillar, or leg. The left erus 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 ver- 

 tebrae. The- not only unite, but they decussate : 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 appen- 

 dices, all converge." 



The diaphragm is the main agent, hoth in ordinary and extraordinary respira- 

 tion ; it assists also in the expulsion of the urine, and it is a most powerful auxi- 



