388 ANGIOLOGY. 



ing proportions. Red corpuscles vary in shape, but in all 

 mammals are more or less discoid, the camelidae excepted, where 

 they are oval ; in birds, reptiles, and fishes, they are oval, and 

 also nucleated. Their average diameter in the horse, ox, or 

 sheep is about TisWth part of an inch, their average thickness 

 being about one-fourth of this. Each surface is depressed 

 towards its centre, hence the corpuscle is appropriately described 

 as a bi-concave disc. 



The white corpuscles are larger than the red, round in shape, 

 and nucleated. Occurring also in lymph they are sometimes 

 termed lymph- corpuscles. 



The liquor sanguinis is pale and clear, and consists of water, 

 fibrin, albumen, fatty compounds, extracts, odoriferous and saline 

 matters. The serum is a thin, transparent liquid, of a pale 

 straw or yellow colour, consisting of the liquor sanguinis deprived 

 of fibrin. It contains nearly 90 per cent, of water, is always 

 slightly alkaline, and coagulates when heated, owing to the large 

 quantity of albumen it contains. Fibrin is a white, stringy, 

 elastic substance, which, when the blood is in circulation, is in 

 solution, and cannot be distinguished from the other constituents 

 of the plasma. 



HEART. 



The heart is -a hollow, involuntary muscular organ, situated 

 between the layers of the middle mediastinum, and in the peri- 

 cardial sac, to a reflection of which it owes its external smooth, 

 glistening aspect. Its form is that of a blunt cone, slightly 

 flattened from side to side, and it presents a base and an apex. 

 The former is turned upwards, and towards the dorsal vertebra, 

 from which the heart is suspended by the blood-vessels that 

 spring from it ; the apex points downwards, backwards, and to 

 the left side, lying at about the level of the last bone of the 

 sternum ; the organ extends from about the third to the sixth 

 rib inclusive. The average weight of the horse's heart is about 

 six pounds and a-half, its length from base to apex about eight 

 inches, its antero-posterior diameter rather less, and its lateral 

 diameter less still. 



The heart is divided by a longitudinal septum into a right 

 and left, or anterior and posterior side. Each of these is again 

 subdivided by a transverse septum into two compartments, which 

 communicate. Thus there are four cardiac cavities, the superior 

 ones, whose free extremities were supposed to resemble somewhat 



